You're welcome! Glad the dimensions work out for you. I'm jealous of all y'all that have a dedicated snake rooms... maybe at my next place.
If your boa's a standard BI I'd anticipate her finishing up around around seven or eight feet. Three feet at two years old seems kinda small for a standard BI, though -- a standard BI fed on the typical 7-10 day regimen can approach four feet by the end of their first year. Either your girl was slow-grown by her previous owner (a good thing!), has more Central American influence in her background, or is actually a yearling. Make of that what you will.
Side note: if you'll be keeping your boa and your royals in the same room, I'd highly recommend having a boid panel and/or IBD test run on your boa. IBD can quickly wipe out a python collection if you're not careful. It's more of a problem on the morph-y to standard boa side of the market, where your girl likely came from. At the very least, a test should be able to give you peace of mind that your girl's healthy.
Four feet in a year? That’d be power feeding, but I agree she should be bigger than 3’ in two years.