I'm not even going to try to figure this one out...who enters their shirt?
One of the meanings of the verb "rentrer" is "to fit in": http://en.wiktionary.org/wiki/rentrer
I've never encountered the verb "rentrer" used like this before. :/
I have never heard this one before either. Is there a French speaker around who can confirm its used in this context.
I just asked a French native speaker and it is correct. You can also use expressions such as 'la bonne taille'
I would like to add: "I fit my shirt" should be added to accepted answers...!