The higher the oz the bigger the glove tends to be, i train with 14 oz gloves so when it comes to fighting my hands are much quicker as you only fight with around 10 oz gloves and when sparring you need more padding so you dont hurt your partner

It depends on the size of your hand. If you have a big hand, then wear a big glove and if you have a small hand ware a small glove or you don't have to worry about thinking and get the "one size fits all" :]

It depends on the gloves. Sometimes the size is not affected at all, just the padding.