it also depends on your body type. the bigger your body the bigger the gun you can carry ( in therory). i just got a glock 19 (15 round mag) and my friend has a glock 26 (10 round mag) and the sizes aren't that much different for the increase in ammo. i'm very new to guns and these 2 are really the only 2 i have any expense in but i really like the glock hand guns for what i'm looking for. one of the good things about the glocks is the magazines are interchangeable. so if you get the g26 for the smaller size you can use the g19 or g17 mags of your backup mags and have the extra ammo holding power. i was going to get the g26 and use the larger mags as backup mags but i found a really good deal on a used g19 so i went ahead and bought it. but everyone has their favorites for different reasons so you'll have to just go try some guns out and see which one fits your needs best.