For Skeet use #9's or #8's (7 1/2's or skeet will work but because the shot is bigger you'll have less pellets to spread on those crossers). For Trap, I've always used #7 1/2's for the better hitting power of the larger shot. However, #8's will work just fine as long as the clay isn't getting out there too far. And for sporting clays, I use a combination of #8's and #7 1/2's, depending on the presentation.
All in all - you won't go wrong with #8 shot. Remember- it's all in your choke tube selection and how far the shots are going to be.