for dawn to dusk use a good 40-45mm is fine but as with most things you only get what you pay for unless you get a really good used bargain.
i have 2 simmons wtc scopes in 40mm and 44mm and they are some of the best scopes i've ever owned.the optics are fantastic(especially on the wtc 4-12x44 which does well past dusk).
if you are on a tight budget then it may well be a better option to buy a quality used scope.