ArmA 2 mission file name length max characters is 30. If you try to save more you cannot, the save dialog wont allow you to type more characters than 30.
Mission editor load dialog seems to dynamically adjust into even long file names which you have created in windows explorer etc by mistake, however this is a moot point because even though you can LOAD those too long file names, when you go into saving them they are cut off at 30 characters.
Mission editor save dialog displays only 29 characters, so the input field moves with your cursor when you type in the file name's last character (heh difficult to explain). Its only the last allowed character which is not shown until you move the cursor over at the end of the line.
In ArmA 3 the basic length is the same, 30 characters, try to save anything more and it gets cut off. However in ArmA 3 the save/load dialogs are very wide which show you very long file names without moving cursor etc.