The size of your training gloves depend on the size of your hand. Remember, we are not talking about sparring gloves here, but rather, gloves you hit the punching bag with.
In general, if you are a woman with petite to average size hands, you will wear an 8-10oz glove or a Small, if you are a woman with average to a little larger hand, you will wear a 12-14oz glove or Medium.
For men, generally, you would wear a 14-16oz. glove or a Large.
You are not really buying gloves that are heavier than you need in order to work out harder. Not necessarily true. Fast hands come from muscle memory. If you have over sized gloves on which make you punch slower, you may end up punching slower.
If your hand is floating around inside your glove, it is too large of a glove.
If your fingers are crammed at the end of the glove, your glove is too small.
You can always grab some Boxing & Barbells gloves which are open palmed and you can make a full fist in. They have all the padding and wrist support of a traditional boxing glove but allow for you to use your hands while they are on. You can LIFT weights, DRINK water, use your PHONE and more!