Mare,
I have found that pizza screens are cheap enough such that I can have several of them. I have the following sizes: 9", 12", 14", 16", and 18". That way, I can use the closest size. If I had to pick just one size, it would be the 16" since that would allow me to make pizzas all the way up to that size. The 18" would be too large for small size pizzas and, while it fits in my oven, may not fit in other ovens.
Peter