For an easy way you can replace one of the shorter characters (e.g. Wee Man, Ramp Kid, Imp) with Rayman. Shorter characters use in-engine scaling, which applies to the replacement character as well. Note that you can't replace Space Monkey/Ped_Dalek, because it has a green color hardcoded into it that tints the replacement model.
You can also do CAS scaling, though this is a bit more difficult. For this, you have to unlock the debug menu using the Anteara mod on a second install of the game. The debug menu will allow you to replace your custom skater with a different character model, and you can scale that character to your liking using the CAS scale options. Note that you have to use a character that doesn't have in-engine scaling already applied, or else CAS scaling won't work. Jesse James and Aborigine are good characters to do this method with since they don't show up in Free Skate as peds.
