According to this YouTube video from Nintendo Mario Maker will be receiving this functionality come Nov. 4th (Nov. 5th in Europe). You will be able to attach powerups together and then place them in a block. By attaching a Super Mushroom to a Fire Flower, for example, and placing it inside a block, Mario will receive the Super Mushroom when he is small and the Fire Flower when he is large.
The pre-update method to accomplish this was like so: When there are already two Yoshis on screen, the next egg to hatch will have a power-up that varies depending on which power Mario already has.
Small Mario -> Mushroom
Big Mario -> Fire Flower
Fire Flower -> Mystery, Leaf, or Helicopter, depending on Game Style
You can throw together a hodge-podge power-up block by walling off a pipe that spawns Yoshi Eggs and placing a ? Block (with a Yoshi Egg inside it) next to this setup. It works better if Mario is already riding Yoshi, or you place an egg below the pipe manually, as the pipe needs only spawn one more onto the screen and there's less chance the player will just end up with another Yoshi from the ? block.