Metal Pizza Peels have a thinner edge, which works better than the wooden peels's blunter edge for sliding under the baking pizza to lift it out of the oven and onto your cutting surface.
A wooden pizza peel works better as a platform on which to build your pizza and slide it into the oven.
It's best to have both.
If you buy too long a peel, you won't be able to use it well in your kitchen.
The maximum size pizza you can make is determined by the size of your pizza peel's blade (the shovel end).