A lot depends on tire brand. For instance, Continental Top Touring 700x32 are 29 mm wide on my rims, whereas the 700x37 are 36 mm wide. And IRC Tandem 700x30 are 30 mm wide.

So:
- will you be riding for utility?
- how are your streets, bridges...?
- how much you weight?
- do you value speed or comfort?

An indication might be the pressure you like. If you can't stand tires at more than 90 psi, then go for 32 mm; but if you like 125 psi tires, go for 28 mm.