Ditto, every 3 days is too often at any age.

Babies can be fed every 5 days for the 1st three months and then should be bumped up to every seven days for the rest of the 1st year. After that you could keep it at seven days or extend it to a 10-14 day feeding schedule depending on how large of a snake you want to end up with.

My males are fed every 5-6 days from their 1st meal up until they're about 3 months of age. After that, as prey items increase in size I'll extend it to a seven day feeding schedule for the rest of that 1st year. For the 2nd year or by the time they move up to med rats, I extend it to a 14 day feeding schedule. At this rate you should end up with a 4-4.5ft male by their 2nd birthday and would continue to grow slowly to 5.5ft, once at that size a medium rat every two weeks is basically maintenance.

I like to keep my males smaller than the females, between 4.5-5.5ft, lean, mean, breeding machines.

Very large and/or obese males may not make very good breeders in the long run, it's not healthy and it's just not needed.

My females on average will keep the same schedule as the males, except will be fed larger meals. Females I will keep on a seven day feeding schedule up, but will move up to large rats and will keep a seven day feeding schedule until they are feeding on Jumbo rats. Once they are feeding on Jumbo rats, I'll extend their schedule to every 14 days. I prefer my females 6ft-7ft, 8ft tops, well muscled, not fat, or huge. I know, most feel the larger the female the more babies and while it's true, the larger females also may take more time between breeding. A 6.5ft female may drop 20-28 babies and be right back at it next year for another 23-30 babies. A 10ft female may drop 30-40 babies, but it will take a lot out of her, she may need that extra year to fully recover.
Just my $0.02.

Of course, this schedule changes for breeding season, females recovering from birth, etc, etc.

Hope this helped.

As for size of prey to feed:
1st, get them on rats asap, boas are not JCP's or BP's that can be a hassle to switch over to rats. Boas are usually pretty good and will take rats as soon as offered. As soon as they can take a rat pup, move them over.

Feed them a size prey that is roughly the girth around their mid section. You want to see a slight bulge after feeding, not too small and not too large.