shouldn't 'little' be correct as well as 'small'?
Correct,and it's been added. ^.^. When it comes to sizes, there is almost no difference between "little" and "small". The actual difference is only when you're describing quantity.
In Greece I sometimes read the warning sign " στένη γέρυφα" = "narrow bridge"
Well, στενή γέφυρα. in Greek translates to 'narrow bridge' in English. It's not inclided in the alternative translaions, because it's not an accurate one. Something can be small without being narrow.^.^
In English we would probably refer to a low bridge rather than a small one