Make the bevel at the width where it will be at least a pixel wide for most of the time it's being viewed. Or, two pixels wide if you think it's not too comically wide up close.
The idea is you want the edge or bevel to have enough space on the UV map that there's enough pixels to describe the edge. If it's too tight it will be a pixel or less and have unwanted artifacts. You also want the edges to hold up when the texture switches to a lower mip. So the tightness depends on the model, texture…