It's an interesting one. I'd definitely say that men other than my husband started 'paying attention' to me when my BMI hit around 28ish? BUT and it's a huge but... I don't think that had anything to do with my size.
I changed to a male dominant workplace. I started going out a lot more, and wearing clothes that I would NEVER have worn before. Nothing 'slutty' mind you, just more high heels and skirts etc, more feminine clothing than I'd ever worn before. Do I think men started noticing me more? Not really, no. I think there's every chance that I just didn't notice them noticing me before then!
My hubby has said a few times that he doesn't notice when women hit on him or are being more attentive than usual, and I think I was the same. I just didn't notice because I didn't think it was possible.
So I guess if you're hoping for a magic number as to when men might start finding you sexually appealing, my thoughts are that perhaps you're looking for the wrong thing. Chances are that there are plenty out there that are already attracted to you and you just haven't noticed yet.