I will agree with the others. With the newer scopes and coated optics, especially the high end ones 40mm should be plenty. I don't think you would be able to effectively use the extra light a bigger scope would have. If you were going with a much higher power it might be a consideration, but not at that power. Like said, I think a high end compact 32 mm would even work well.

I have an alaskan guide model 6.5x20 in a 44mm and it is plenty. And it doesn't hold a candle to 600-1000+ dollar plus scopes in clarity. I mean it is pretty darn nice for a sub $400 scope (370 something I think?). But when you step up to the next price range there is a noticable difference. At least there was to me when I looked at them.

Paul