"Je rentre dans ma chemise."

[sierradante]

I'm not even going to try to figure this one out...who enters their shirt?

[riclage]

One of the meanings of the verb "rentrer" is "to fit in": http://en.wiktionary.org/wiki/rentrer

[Laetitia_Lalila]

I've never encountered the verb "rentrer" used like this before. :/

[fraserbooks]

I have never heard this one before either. Is there a French speaker around who can confirm its used in this context.

[Chefie]

I just asked a French native speaker and it is correct. You can also use expressions such as 'la bonne taille'

[narko]

I would like to add: "I fit my shirt" should be added to accepted answers...!