I don't think there is a hard and fast rule about BB size, as it depends on variables. The width of the BB, the clearance of the chain stays are the two that come to mind, and then the crankset itself will want a certain length.

I would be surprised to learn that a component manufacturer can specify a BB size and have it work on all random frames. Logic dictates otherwise.

Personally, I would check the fit of what you have and if the crank fits fine, stick with that. If the chainline is off and you have the clearance, then you could use the old one as a starting point and measure from there.