It doesn't seem at all unreasonable to ask a prospective hiking partner about their physical condition. The rub is that often the least able hikers are so ignorant about hiking that they can't even give a reliable assessment of their ability. This is not a criticism, it's simply a fact.
Of course, gender and weight issues are only part of whether a person will be a suitable hiking partner. I've had some unfortunate experiences with hiking partners "met" online...and I've had some good ones, too. My best advice is to plan something very short and simple, with good bail-out options. Don't share gear, and make sure your partner understands that, if he/she quits, you are planning to go on, and you expect the same from them.