The obvious answer is that there is no such thing. Everyone has a different idea of what an optimal tire width and that has a lot to do with the kinds of roads you like to ride (paved only or paved plus gravel) and the amount of weight you carry (minimalists can use skinner tires).

But what if you're not a minimalist in terms of weight and you like the idea of doing a gravel road from time to time?

The old standard for touring was 700 x 32c (or 27 x 1 and 1/4). Road racing tires have been getting fatter and so have touring tires. Some very good touring frames can't handle much fatter than a 32c and fenders, some can.

I'm thinking that a touring bike that can handle a 700 x 38c-40c tire (or the 26 inch equivalent) and fenders is probably optimal if you're not a minimalist in terms of weight and you like the idea of touring on gravel roads.