Weight itself wouldn't make a huge difference, it'd be more bodyfat levels if we're speaking technically on what I'd care about. An example would be like, Lydia Valentin, she's an Olympic weightlifter, she's 5'6, not too far away, and pretty great looking really. But she's quite heavy by most girl standards, she competes in 75kg class, so she's probably 160 pounds or so, but she doesn't "look it" at all. With the Lydia Valentin though, she's quite hot, http://www.flickr.com/photos/dreamsplayhere/3602041995/ , http://www.flickr.com/photos/wonderlift ... otostream/ quite well proportioned, but she's "overweight" by about a point according to the BMI. Did I mention she's hot? Anyway, BMI totally fails in cases like that, where people are either by training or genetics just more muscular.
As far as weight goes, it's pretty much only fair to expect a girl who's around my size or so. It'd be hypocritical of me to say I only want a super skinny girl, if I myself have some bodyfat on me. So there's that, can't be a hypocrite, you see this a lot online, like 300 pound guys who'll say a girl who's not really "fat" at all is fat, when they're 300 pounds. Hypocritical to the max. In general, I think it's only fair to expect a girl who's about as good looking as you are, with similar proportions as you (not exact obviously, as we're biologically different and all, but I hope you can get the idea I'm trying to get across.)
As far as personal preference, I don't know, I tend to enjoy women that are curvy, but curvy doesn't mean a whale, or obese either. Probably if we are using BMI, as flawed as it is, 23-25? Maybe 26-27 if the girl pulls it off well, that's the deciding factor, you know? If some stick thin girl liked me, it's not like I'd be like "nope, too skinny, next!" or anything, if she liked me and things worked out and all that, it wouldn't be a deal breaker, but yeah. Too fat would likely be a deal breaker, possibly because, first off, as you marry, girls don't tend to get skinnier, and second off, it shows a lot of problems in lifestyle choices and stuff like that most of the time, and it'd be a clash in values, besides not looking good to me. But if I could just draw a girl out from scratch as my ideal mate or whatever, yeah, those would probably be the numbers. Also, not to do with weight, but tall girls are sexy, too.
Damn, I made this post too detailed and sciency. Oh well, pics of Lidia Valentin then.