Can a medium creature with Friendly Toss and Titan Wrestler use the 2 action Friendly Toss to "toss" an ally who is size large or huge?
No as the Friendly Toss power limits it to an adjacent ally of your size or smaller and is not technically anything to do with athletics.
That sort of limit would not be unreasonable for a GM to relax.
Some Barbarians have ways to get larger.