I think it's more to do with the proximity of the stands to the pitch than the size of the pitch itself. And to pull the pitch back far enough away from the stands would render it too small for use.
There is certainly a minimum pitch size for European games though and it's bigger than the minimum pitch size for domestic ones. It's one of the (many) reasons why Queen of the South couldn't use Palmerston park for their brief European foray this season.



