Well, I don't know the answer so I'll just speculate.
My feeling is that the tube is more rigid than the dovetail and the longer the dovetail the more chance it has to flex. So shorter may have advantages. The downside is that you have less balance adjustment with a short dovetail (unless you move the tube through the rings). Having said that, I have a longish dovetail on my 15cm refractor and I'm blissfully unaware of any flexure problems.