There are more variables than that. Speed and the style of flying are big factors too. Honestly though, most servos are significantly stronger than they need to be. .60 sized aerobatics planes were flown in the 70's with 25 oz in servos that were huge and heavy. Now a 1 oz mini servo will put out 70 oz in if I choose to spend the money on one of the better ones.

For .40-.60 size trainer and sport planes, you can't go wrong with a standard size analog ball bearing servo. Specific ones include the Futaba 3004 and Hitec 425.