Keeping your horses mane in small, or even larger braids is fine. During the summer is it not uncommon to see ALL my boys with braided manes :) it just helps keep them cooler, and their manes cleaner.
The size of the braid also depends on the horse. For my most impatient, mustang-like three year old, I do larger braids. His whole mane is contained in maybe 4-5 braids. It is VITAL that these braids are kept LOOSE at the top and that any pulling hairs are corrected after braiding. He's had his braids in for a good 3-4 weeks now, with no problems and a much cooler horse.
I do more medium sized braids on Sammy, my 16 year old. He has a pretty standard mane and it's really just to keep him cool during the day. He has some skin problems and so the less he sweats, the less he itches. Maybe 10-15 braids. I redo his braids about once a week since he scratches and messes them up so much :) Again, these braids are loose at the top so they don't add to his discomfort.
Loki gets the smallest braids. I braid his mane for both cooling and for pretty waves. He gets 15+ braids, depending on how patient we both are, and how I want his mane to turn out wave-wise. Since his mane is so long I often leave a few inches straight at the bottom. When I take out the braids it looks awful for maybe two days, then the waves relax and it's rather pretty.
His sire has a naturally wavy mane (very, very pretty O_O) that he unfortunately did not pass on to Loki, who has a very straight mane like his mom instead >.>
Wait! I'll fix it....