i use a set of 12oz gloves, these ok. Also what are some of the best exercises to enhance hand speed

Well i usually used my 10 oz green hill gloves for the bag, but started having some hand problems so i just got a pair of 16 oz winning gloves and they are great.

For the right ones i guess it usually matters on your weight alot of the bigger guys use 16 oz at my gym and all the smaller guys like myself use 10 oz, i chose 16 oz just to help with arm indurance and stamina..
and for enhancing your punching speed i just shadow box with 1kg dumbells and use heavier gloves, kinda makes a difference.

It depends how hard you hit,like for me I use 16oz and above on the bags because anything less hurts my hands.