Snakes go through growth spurts where they want to eat a lot and grow fast. Other times, they slow way down. There is nothing wrong with stepping up the size as long as its within reason. Like I said, a noticeable lump I find acceptable but something that looks like it ate a football and has to be rolled onto the hot spot would be a bad idea. I kind of can read my boas and know when they are hungry as they pace and push and are very active. This usually happens around the 2 week mark. I couldn't go a month as they would break down their cages and probably try and eat meAlso remember, females generally will eat more than males. Most of the people I've talked to as well as the Boaphile feed their males once a month but females get fed every 2-3 weeks. I do generally lean on the smaller side as like I said, I feed every 2-3 weeks so usually my 6-7' girl gets a large rat with a small rabbit thrown in every few feedings. My 5' sunglow gets a full sized medium rat every 2 weeks. The BP is scared of medium rats lol so she gets a small every 2 weeks and the carpet and baby dumerils get a small adult mouse and mouse hopper respectively every week.
Like I said before, give a medium a shot. Cant hurt to ask the feed store to let you check out their small mediums or to pick the smallest medium and then check it out and see if it looks about as thick as your boa. If they all look too big, then stick to smalls and try and find large smalls.
Lol the only ones that wont eat in shed are my carpet and the BP. Everyone else fine with eating while shedding. Heck I bet they would eat while pooping if I had a rat handy the few times I've caught them dropped a deuce lol.