If you're building a cruiser than to me, the biggest concern aside from cost is sound. I bent custom pipes for a couple years and I'm particular about sound. In terms of performance in a naturally aspirated engine - there is never a "loss" of power when less backpressure is introduced. Not ever. But - if your engine isn't moving enough air to over-fill it, or the manifolds are more of a restriction (any upstream issue that causes more restriction than the pipe size means the pipe size becomes irrelevant), you may not ever notice a gain. As an example - a 2bbl 383 with a Y pipe and 2" tail pipe has virtually no exhaust backpressure at idle. Bring up rpms and it begins to overfill the exh system starting with the Y pipe and you get backpressure. Replace the Y and single pipe with 2" duals and you will get a gain in power that will be slightly offset by additional weight of the system. Replace the camshaft and you will get more of a gain just because the better exhaust lets the camshaft work. Right up until the manifolds become the restriction. Replace the log manifolds with HPs or headers, and it will realize a smaller gain because now it's the pipes and mufflers.
So - in terms of a recommendation for you - 2 1/4 will work and if you have log manifolds should be fine. If you have HPs or headers I'd go 2 1/2". The H or X pipes will even out and quiet down the tone and trick the gassses into thinking the system is bigger than it really is by harnessing the physics of pressure ways moving down an enclosed pipe. They are labor intensive if it's a custom shops and they add weight (although not much). IMO it's not needed but if you want it, or want to run the 2 1/4 with HPs or headers, go for it. Lastly - if you want sound - a larger diameter pipe will be louder at a higher rpm than a smaller one, all else being equal. I used to like the sound of 2 1/4 head pipes going into high flowing mufflers and a 2 1/2" tail pipes on a mild big block.