The difference in size is due to different bore sizes, as well as different stroke lengths on the crankshaft. The size on an engine is rated in cubic inches(or liters on newer engines). This is a combination of how many cubic inches measured in the bore, including the diameter of one cylinder bore, and the length of the piston stroke. That figure is multiplied by 8(on a V8 engine) to arrive at the final cubic inch rating.
(I don't have any specific figures handy, nor am I intelligent enough to have them stored in my pea brain)
The bore size is the diameter of the piston(plus a little for clearance), and the stroke is how far up/down in the bore the piston moves, which is a function of the crank.
A 355 is a 350 that has been bored(oversized) .030".
A 383 is a 350 that has been bored, and has a special crank that gives it a longer stroke. This requires a special rod length, as well as some minor block grinding to clear the longer throws of the crankshaft.
The heads, valve size, and cam, have nothing to do with the cubic displacement of the engine.
Hope this helps some...

383s rule. If you want a hot rod!

I heard that! I've driven a few cars with 383s in them, and they were screamers!

Thanks for the info!

So can I have my 350 bored and stroked to 427, or is that a big block?

Well...that's just sillly!
Actually, there are physical limits as to how far you can go on a small block. I'mma thinking the 383 is all she'll do....think about that for a second. You've added 30+ cubic inches!
You CAN go 427 using a bored 400 block, and a stroker crank, but from what I've heard, those are not very bulletproof. The cylinder walls of the 400 are pretty thin to start with.