In the full-scale world, winglets are designed for a single speed, usually to reduce drag at cruise, where most fuel is used.
For a model, this is seldom of importance.
About all you could do is try some, varying the sizes and angles, and comparing the way each feels in flight.
There's no real quantitative way to check the effect on a model, it will be up to you to decide what you like.
There won't be any serious bad effects in any event.