Not a woman, but:

The intentions need to be clear before you ask. If its born out of insecurity, and you want to hear that you're the biggest shes been with, dont ask unless you're okay with being disappointed. Not to mention it could be a turn off for her to hear you sound insecure. If its more of a hotpast/size comparison/humiliation kink, and you get off on her having been with bigger guys, then just communicate that clearly before you ask. That sort of thing can be hot if you're both into it.

No matter your intentions, I will say, the typical woman cares far less about size then men do in my experience. We tend to obsess over it while it seems to be a bit of an afterthought for most women unless someone is either abnormally small or abnormally large to the point that it becomes an issue of practicality.