Depending on how your BB is constructed, reinforced, whatever it will fly well on some .25s (Especially with a smooth hand launch), no problem with a .29 -- .35, o/p with a .40 and I certainly would not go beyond a medium type .46. A 40-46 might need a close look at the wing interior bracing.

Flew a BB back in the '70s. It was an older ST. 40. Belonged to my friend and he let everyone fly it. In the past 7 years I have had 3 "Old Timers" of the 70 -75" span. The first two were well over-powered with an Enya .29 and the last one was with a ST. 29. The first two were monokote and beefed up for RC. The last one was silk and dope. All three suffered multiple fatalities before a final, simply because of many pilots (including newbies) getting into situations and over-stressing the wings. That is where the OTs fail. IIRC, the BB is a bit stouter than those I had.

It is so graceful to get one all trimmed to fly medium circles, set the TMTR down, and when it slowly drifts away, retrim to the other direction in a much larger circle and watch it slowly fly itself back - no / low wind days
When it's all trimmed up to do that, give it to a newbie and in 10 seconds he's upside down. Proves just how it got there.

Good Luck and have a ball.