heck, i think most 4 engined strategic bombers (think b17, b24, lancaster) should only be available in large airfields
and 2 engined tactical bombers (think ju88, b25, g4m, mosquito) should only be available in large and medium airfields
and single engined tactical bombers (think ju-87 IL-2, TBM etc) should be available in large/medium/small airfields