What weight class are you?
I'm 6'3" (186cm), 220lb (100kg). A high-quality, leather, 10 oz, dense foam, full-wrist velcro-fastened glove suits me well and lasts approx 2 - 3 yrs of 3 training sessions of 6-9 rounds of bag work per week (Approx. 2,700 rounds without injury), and about 1000 rounds of light sparring.
If you are much lighter, 8 oz gloves will likely be fine. If you are heavier, 12oz might be suitable. 2 oz to either side of 'ideal' should not present too many issues.
A downside to using bigger gloves for bag work relates to leverage. A very wide glove can lead to your wrist sustaining lateral torque injuries when hitting the bag hard, usually during hook-related strikes, where it is common for the leading inner edge of a big glove to contact the bag first. Hand size relative to glove size is important here. If you have an extra small hand, then the torque exerted upon impact by a bigger glove will be greater, and vice versa.
Remember too, that glove weight does not always equate to glove size. Density of foam/horse hair influence how big a glove is relative to its weight. In my experience, better quality gloves are constructed more densely, which results in a smaller glove per oz.
Heavier gloves also mean that less power from your punches is transferred to the bag. This means the bag will move less. This can be a good thing, or a bad thing, depending on your training goals/style and the weight/size of the bag. If you are hitting a very dense bag, heavier gloves may help prevent repetitive stress injuries.
Heavier gloves can be a great way to increase you local shoulder-muscle endurance and even aerobic endurance, and prepare you better for sparring with heavier gloves, although this is outweighed - in my opinion - by any of the aforementioned wrist-sprains that larger gloves can cause. There are other ways for you to build your endurance.
Also, don't limit your search to 'bag gloves' or 'bag mitts'. Many of the gloves advertised in this way are simply 'lesser gloves' that don't have nearly the quality of support, fit and durability of proper boxing gloves.
All this being said, you will likely adapt to whatever you can afford/is available. If possible, try some gloves at a gym prior to purchase. Many gyms have plenty of different gloves to use free of charge. This experience - coupled with advice from any decent trainer who can assess your needs in person - should point you in the right direction.