Boxing Gloves 101

I go to a weekly cardio boxing class at my gym. I wanted to get my own pair of gloves for light bag work and mitt sparring when I am not in class. Everyone in class uses the same gloves, so I am assuming they are all L/XL. They stay on with wraps when the Velcro is tight, but I have a lot of room. Do females usually use the small/medium size? In normal gloves, I wear a medium. Also, what effect do different weights have. The smaller sizes on Amazon (in the $20-$40 price range) have lower weights. Is that okay? Is there anything else I should know? Any recommendations? I do not actually attend a boxing gym, just a gym that offers boxing as part of their IT classes. That is why I am not just asking at my gym for recommendations. Thanks in advanced!