If it were me I'd get in order of usefulness #6, #4 and maybe a #2. The smaller you get the closer you get to an exacto. Most people think of a chisel and smacking it with a hammer, but that's not what they are used for the majority of the time. There are lots of different kinds of chisels, if I could only have one chisel for this hobby it would be a 1" paring (patternmakers) chisel. It has a very long thin blade and is used for carving, almost like a plane but without the body and cutting end grain.

The picture below is one example of carving the nose on a P-26 from a balsa ring. This is one place the paring chisel excels because you can place the flat bottom on a surface and carve forward of that. On the P-26 the fuse was shaped, then the balsa glued on oversized for the nose. By placing the flat bottom of the chisel against the fuse I could quickly, and without creating a ton of dust, carve most of the wood to shape. With a chisel it is much easier than sandpaper to see and control the shape as you carve, and on something like that nose ring where the wood grain is going in just about every direction, it will cut in many directions that a plane simply will not, crossgrain, endgrain, etc. Look at the nose ring above where I'm carving, that's how close you can get it with just the chisel, then just a couple minutes of sanding to smooth it out and done.

I've attached a few other pics from the p-26, all the blocks on the wingtips, stab, elevator, fin, rudder were carved with the paring chisel and all the blocks that were part of the wing saddle too. Look at the pic of the wingtip with the block still not carved. Picture laying the chisel across the wing and carving the wood down to the level of the wing. With the chisel laying on the wing there's no chance of taking too much off. One last thing, the handle on a paring chisel is nice to have, once in a while you even use it, but most of the time you just hold the blade. In the first picture I would hold the blade in front of the handle with my right hand and push it away from my left hand with my thumb to make a slicing motion.

The base of the fin and rear of the fuse is one balsa block. The chisel was used to follow the lines of the fuse, then I drew the outline of the fin and used the chisel to cut straight down. When it was close I glued the fin on and using the same technique from above, layed the bottom of the chisel on the fin and pared straight down. You can see the results in the second picture where the fin is glued on.

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