Here is another post I will share with you this might answer some questions that you might have ....
HAND WRAPS MUST BE WORN AT ALL TIMES!!!
SPARRING;
OK, sparring, depending on what you are attempting to do, there are a range of gloves you can use...
8-10 oz... this is used for technical sparring where speed and precision is the key. The lighter gloves ensure you can throw faster punches and throw many more punches without the worry of tiring. Good for timing and accuracy, therefore used for offensive fighting.
12-16 oz... these sized gloves are used for hard workouts, where both fighters are required to fight pretty intensely... The idea is to strengthen you defense and give you a fairly limited options in punching as you are slower you have to pick your shots making you far less vulnerable to punches, good for defensive fighters and is used for strength and endurance.
18 oz +... these are worn when both fighters need to be challenged both mentally and physically, the size and weight of these gloves makes it very difficult to work hard for long, so therefore these gloves are usually used for big hitters, to produce great strength, and great muscular endurance.
HEAVY BAG;
The hardest exercise a boxer can execute, whilst strength and endurance are important in having a challenging and effective workout, the boxers hands must be protected also...
For heavy bags it is suggested that you use 14 oz+ gloves, (handwraps are a must so you don't snap your wrists) this is because the force of a punch on a heavy bag needs adequate protection so that your hands do not get cut or injured... Some people say that you should use the heaviest gloves you can find but i believe this is rather unnecessary as you cant punch anywhere near as hard as you can with a 14 oz glove. 14 oz gloves offer protection at a fairly high level which is why you should not go below that weight (unless your doing light, quick punching drills, in which case you can use lighter gloves). Glove size is important as the heavier you go the better protected you will be, but you get tired much more quickly.
PADWORK;
It has long been a rule in my gym that you do pad work with the gloves you spar in... This means practicing with 16 oz gloves on the pads would have you sparring in 16 oz gloves.
This is just a BASIC guide for gloves but it is very accurate, if you have any doubts, try a few drills with gloves before using them therefore you don make a mistake of realizing you're wearing too heavy gloves in a 3 minute round 1 minute in... So yeah I hope this helps.

If something doesn't sound right feel free to correct it.