Wow... A lot to respond to here haha. Let me know if I miss anything...

Snakes really never stop growing and sexual maturity can vary. Some female balls can breed as early as 18months and some may take up to 4-5 years. Same goes for size. Some females are sexually mature and lay clutches at 1300g and some get up to 5kg monsters. I'm sure the same variances apply to all species of pythons. There's no real SURE age or size where they're full grown and sexually mature. I can speak on boelens specifically because I'm unfamiliar with them. Not sure what their metabolisms are like at all. If they're in the morelia family I'd say feed something that leaves a small lump in her tummy every 10 days. Carpets seem to have a bit of a slower metabolism, guessing it may be similar for boelens. A good way to tell would be just like you said, just eyeball it. Feed something about the same size as her widest part and see what kind of lump it leaves.

I'm not sure what you mean by under and over conditioned.

You can feed two smaller rodents or one larger one. Really doesn't make much of a difference. Sometimes snakes will be picky and refuse to eat two so it's more convenient to just feed them one bigger one. Just depends on the snake really.

I would say your gal, at 3 years old and 6ft, is considered an adult. As I said, every 10 days would be an ok schedule for her. Every 12 would be ok too though. Schedules are for the keepers not the snakes.

I don't have a problem typing on here but the forum has been "unavailable" and giving me error signals pretty frequently lately whenever I try to log in on Tapatalk. It's rather frustrating