there are a few boa regulars who must not be checking their phones today, but you are going to get a lot of good info on this topic in this forum!
The long and short of it is, less is more, it is very hard to underfeed a boa but very easy to overfeed. as far as size of prey and schedule, variation most closely resembles their natural habits. sometimes a small item sometimes an appropriate one (same diameter as mid-body) and dont abide by a set schedule. frequency does depend on age but anything over a year should be fed twice a month at the most, but less is fine, and certainly less as they get older. that is actually a rather liberal guideline, many fast their boas over the winter, or reduce feedings to every 6 to 8 weeks with very good results; increased activity and energy and, perhaps counterintuitively, strong growth. My boa just turned 2 so this winter will be his first fast.
Like i said, this thread is very likely to yield a ton of very insightful responses, so stay tuned.