There are a bunch of formulas around which will help you, but it's important not to go too big.
You need as little backpressure as possible, but you also need proper gas velocity for low speed torque. If power is the only option, you can have 1 square inches of pipe for every 35 cubes of engine. If a quiet streeter with a nice purr is your goal, then 1 square inch of exhast area per 45 cubes.
On a 200,
twin pipes with a minimum internal diameter as small as 1.675" for quietness, or up to 1.875" for maximum power. If you go bigger, you won't get any extra power, just a bigger rumble! In Aussie, worked 202 Holdens have run a single 2.25" pipe for years, and thats 1 square inch of pipe area for ever 45 cubes.
If you are going ballistic, a set of twin pipes with internal diameters of 2" are about as big as you'd ever want. I see no precendent on any legal normally aspirated car with overall pipe areas of less than 30 cubes per 1 square of pipe area. I've not seen a 400hp 5.7 or 429 hp 5.4 V8 run twin 2.625" pipes, for instance.
A cheap set of 289 or 302 V8 duals should be all you need.
Read
wsa111's speal on cross-over, X or H pipes on sixes. Japs and Germans have been doing them for years.