My tag says 5-6' tall & wide. Zone 5 through 9. Proven Winners site says 48-60 inches which is close.
Don't think I've had 'Royal Red' but I've never smelled a Buddleia I didn't like.
It would be heartbreaking to lose them all & I only did 1 time. 'Miss Molly' is the only one that didn't show signs of life this spring until very late. It is growing pretty good but I think many are very behind this year for some reason. I normally would have flowers on most by now & only some are just budding. 'Groovy Grape' was new last year for me & it is budding the best & near ready to flower. 'Lo & Behold Blue Chip' is very behind but foliage looks great. Get JB's on flowers from time to time but not terrible. They aren't flowering yet so JB's are seeking the Cupheas instead.
I wouldn't care if Buddleia is not native. Many non-natives are beautiful. It would be invasiveness that I would worry about & don't have to where I'm at. I rarely get a seedling but now have only 2 in 20+ years. I was just at a plant seminar this spring who said to cut them back in fall & I highly disagree. I have never & only cut back to about 2-3" in mid-spring. I've never transplanted one to know how they do but I would hesitate to do that except only in spring, especially when most say Zone 5 on the low end & that is my Zone. Lack of snow along with very cold temps is the reason to lose them, I believe.