Fitting inexpensive hybrids is pretty straightforward, and you can generally use the XS-S-M-L-XL-XXL designations that come with the numeric size (which probably doesn't correspond to any actual dimension of the bike, anyway. Just remember that for men, medium means around 5'7"-5'9", for women, it's around 5'5"-5'6". On these bikes the reach to the handlebar is shorter and higher, but you want to keep the same reach from the saddle to the pedals. The riding position is a lot more upright, so issues like athleticism, flexibility, and general fitness are less important.

One thing I see is riders choosing hybrids that are too large for them because they want a higher handlebar. They often end up sitting too far forward to reach across to the handlebar comfortably. The correct way to do this is choose the size that is right for your height and select a stem, whether fixed or adjustable, to put the handlebar where your hands want to find it.

And don't forget saddle setback and leveling. Bike sales guys tend to ignore this on cheaper bikes, but it's important for keeping you balanced and reducing fatigue. Keeping the nose up a degree or two won't pressure the groin because you're not trying to to rotate the pelvis, an a little setback allows you to lean forward a bit with a straight back.