Large enough to keep the rim off the ground is optimal. I know that sounds snarky and unhelpful but I've been round and round with this question, and eventually I come to: the tire width within reason does matter all that much for speed.

My larger tires have typically been designed more for durability, or flat protection, or even grip, and are hence heavier and slower than my narrower tires which are typically more performance-orientated. So it's tempting to say that narrower tires are faster, but the fact is the tires are faster period. Less to do with width.

I don't want a big block in the wind or the feeling that I have to push it to spin up. But seriously, we're only looking at 6 or 7 square inches more, and given all of the things on my bike or body that take up an extra 6 or 7 square inches it's hard to take the aerodynamic penalty very seriously. So given a particular brand or style of tire, and the particular surfaces I roll on, at enough pressure that it's in that smooth rolling range, it just needs to be big enough to keep the rim from bouncing and that's it. Anything from 23 to 32 is optimal for me, if it's the right tire.