Changing a crankshaft involves a lot more than just knowing "what size".
If you intend to "stroke" the engine (increasing the stroke to increase dislacement), the pistons must be changed unless you can use the Chevy 5.565" rods (not recommended). You Can put a 3.75" stroke in there by using the 400 rods, but the rod/stroke ratio is woefully low.
You would be way ahead to buy a good (forged steel crank, not cheap) rotating kit from a supplier such as Eagle Specialty Products. The kits contain crank, rods, pistons, rings and bearings, for the specific application. Avoid the cheap (under $300) Chinese cast cranks. We've seen nothing but grief from them when trying to make power or rev. They're fine for a pickup truck that won't see 5,000 RPM. We're talking a "383" here.
Another approach is to get a Chevy factory 400 crank (the only decent "casting" out there with this stroke), turn the main journals down to fit in the 350 block, and piece a kit together.
Be sure you use a 4-bolt block, too. Chances are good that you're block IS a 4-bolt, but make no assumptions. One must actually pull the pan and LOOK. We've seen Chevy casting numbers all over the map regarding the 4-bolt thing.
There's a significant amount of grinding in the crankcase, to get clearance for the rod bolts. A "small base circle" cam is also recommended, as the rod bolts can "clip" the nose of an aggressive cam.
This is really a job for a professional engine builder, unless you have mass experience and a good set of machinist's tools. Building a stroker is not the same as an overhaul!
Jim