A- Bandai Full Color gashapon (comes on a round base), they scale pretty well with each other
B- "Ramen" model kit (Lunch is original color, Yamcha has paint on him), there is no scale, they are all just pretty much about 1 inch tall while accessories they come with are half that size
C- Mini Figure Series (MFS) bottlecap figures (come on a round base that looks similar to the cap of a water bottle), they don't have a specific scale, they just are a size that fits on the bottle cap
D- Funimation vending machine figures (the junky toy machines they have in the entrance of Walmart and some other stores), they don't have a scale and are smaller than 2 inch scale
E- AB 2 inch figures, most scale with these, but some of the smaller characters like Uub, Garlic Jr., and Chaozu are close to being HG scale because they are larger than they are supposed to be
F- Bandai Full Color Battle 2 inch scale figures, blend perfectly with AB 2 inch figures but are actually better detailed
G- Bandai Soul of Hyper Figuration, SOHF, they mostly scale really well with each other, they come on an oval shaped base
H- MGA Pinball machine figures (there were only 6 and came with the game), they have a green hook on their feet you have to cut off, but they blend right in with AB 2 inch figures
I- Ultra Figus from Argentina, I only own one of these, but he's painted similar to AB and the same scale. Some of their figures are a little too big and others too small. There's no particular scale among them.
J- AB Combat Set, these are larger scale than AB 2 inch figures and do not blend in despite being painted exactly the same way
K- Bandai Imagination, these don't have a particular scale among each other, but the characters are typically a little smaller than 2 inch scale
L- Megahouse gashapon, these also don't have a scale among each other, but I think they tend to be a little larger than Bandai Imagination?
M- Hachette from France, I would call them 2.75 inch scale but some/most of the characters fit in with HG figures really well, this Tien just happens to be a little small, which is unusual
N- Bandai Hybrid Grade (HG), they mostly scale well with each other and are pretty much 3 inch scale
Edit/Addition 5/30/2014: Here is a Giochi Preziosi Tien/SS Vegeta 2 pack next to an AB Combat Set. I thought these would be the same scale but they aren't. The Giochi Preziosi figures are bigger than AB 2 inch ones, but smaller than AB Combat Set ones. Perhaps 2.25 inch scale while AB Combat Set figures ("J" in the pic above) are perhaps 2.75 inch scale.
O- Family Mart gashapon, these don't have a scale much like Megahouse and Imagination don't
P- Bandai America '96 2.5 inch figures, these don't have a scale and most are 2.5 inches tall
Q- Robust, these are (as far as I know) identical rereleases of Ultra Figus figures, not all figures were rereleased, in fact I think only about 8 were, and they come on these orange 'bottle caps'
R- Popy Mini Big Head, these are all pretty much 4.5 to 5 inch scale heads on tiny bodies
S- Bandai Chara Puchi, these are all pretty much 5 inch scale heads on small bodies, but characters with large hair don't have hair large enough to technically work on 5 inch scale bodies
T- Banpresto Dragon World Collectible/World Collectible Figure series, DWC/WCF, these are pretty much 2.5 inch figures with heads ranging from 5 inch scale to maybe 9 inch scale (mine in the picture are bootlegs)
U- Bandai Ultimate Collection, 3.75 inch scale like GI Joe and Star Wars
V- Irwin Secret Saiyan Warrior, 3.75 inch scale
W- Bandai Real Works, typically 4 inch scale
X- Legend of the Saiyan, typically 3 inches tall and NOT scaled. Some blend in with HG, while others like this one blend in with Real Works.
Y- I can't remember what this line is called, but there are small figures that come with unpainted gray model kits you build (Frieza came with his pod, Pilaf comes with his robot suit, etc.)
Z- Bandai Hybrid, they are "sort of" scaled at 4 inches, though the Gohan and DBGT kid Goku can blend into a 5 inch collection
A1- Burger King pewter colored figures, they are between 4 and 5 inch scale depending on character (Krillin and Gohan can pass for 5 inch scale, the rest cannot)
A2- High Spec Coloring Figure, HSCF, this one is a bootleg, but most of these are 4 to 4.75 inch scale. A few of the characters are 5 inch scale though, such as all of the Vegetas and Frieza form 1. To be honest, their scale isn't very strict, because characters who would need to be enormous are very small (Imperfect Cell, Frieza form 2, etc.)
A3-Irwin, 5 inch scale, most of the figures are 5 inch scale, though smaller characters were made too large
A4-Jakks Pacific, 5 inch scale, most of the figures are 5 inch scale, though smaller characters were made too large
A5- Bandai Super Battle Collection, SBC, 5 inch scale until DBGT, when they get to DBGT the scale jumps to 5.5 to 6 inch scale
A6- Banpresto DX, this line of DX figures vary between 5 and 6 inch scale (Fat Buu and Piccolo can work in a 5 inch collection, this Goku and Gohan in kai outfit cannot because they are too big)
A7- DeAgostini 5 inch scale figures, very few of the figures don't scale well
A8- Editions Atlas 5 inch scale figures, they tend to be a little taller than the DeAgostini versions, but that isn't always true because their Tien is much too small
A9- Bandai Super Battle Collection, SBC for DBGT. Technically these are bootlegs I repainted. The real ones are 1% to 5% larger and very obviously 5.5 to 6 inch scale
A10- Jakks Pacific DBGT figures, they are 6 inch scale and will not blend in with Jakks 5 inch figures mostly because of their much larger heads
A11- AB Super Guerriers, 5 inch scale
A12- AB ~2003 higher articulated figures (Bulma is a bad example of that), 5 inch scale mostly... Korin is much larger than that, and theres some other variation in there too, like Raditz being too short
A13- Jakks Dragonball figures, determining their scale is difficult, I'd say Krillin is 7 inch scale and General Blue is 6 inch scale with an undersized head
A14- IF Labs/Giant Ape Dragonball figures, small characters are enormous, maybe 9 or 10 inch scale and scale well with each other, while taller characters scale well with each other at about 7 inch scale
A15- Academy Model Kits, scale at about 7 inches with Goku and Krillin being more like 8 inch scale
A16-Banpresto Scultures, these don't have a scale at all, so they vary a lot. They are typically around 5 inches tall at minimum
A17-Banpresto Pichi Pichi, these blend pretty well with the 7 inch IF Labs/Giant Ape DB figures
A18- High Quality DX, HQDX, these are typically enormous, but Dodoria was made at 5 inch scale. I don't own others, but I'd say they are around 7 or 8 inch scale typically
A19- IF Labs/Jakks Movie Collection, these vary a LOT, but they are usually between 9 and 11 inches tall I believe
A20- Vinyl Garage Kits, these usually say what scale they are... I think Piccolo is 1/6 or 1/8...? I can't remember. Chaozu, I have no idea. They vary a lot.
A21- Bandai America '97 16 inch scale Goku, mine is a bootleg, but I believe it's the same size as the original
A22- AB 16 inch figures, they aren't scaled figures and don't correspond with each other like scaled figures. They were just made BIG! As you can see, Janemba is only about 10 inches tall