What size should I pick?
The Pizza Steel should be placed on your baking wire rack or baking plate
Measure the length and width of your wire rack/baking plate, then pick the largest size that would fit onto your wire.
PS: If the flat area of your baking wire rack / baking plate isn't large enough, you could try turning it upside down. This often results in a larger area in which you could place The Pizza Steel, since many baking wire racks / baking plates have bends and angles on the upper side making the flat area smaller.