Crash; Selecting the prop you are going to use is a trial and error thing. There is no one size fits all. Each different plane needs a different prop. Be aware that if you are running a large prop for the engines ability you need to be able to tell when an engine is running hot. A smaller prop reduces overheating posibilities to some extent and is advisable if you have little gas experience. With experience your ear can give you a good idea of when an engine is running hot. Like Dick said above he runs a prop he's decided to run because he has experience. But he'll be the first to admit sometimes your first prop pick isn't the last. So pick a prop, use it, see if the engine is turning in the rpm range you expect, and make sure it isn't getting hot. For DA 50 you'll be somewhere between 22x8 to 23x8 for aerobatics. Maybe a thin 23x10 for scale non aerobatic and very little WOT. A phone call to DA is a better place to learn what fits best. You will learn with time how to select props.