In most bird raptor species, the female is larger than the male. This goes against conventional biology logic for nearly every other bird/mammal/lizard species, but it is indeed the case assuming you're basing T-rexes off a raptor.

But you were probably thinking of velociraptors. Given the initial size of T-rexes, there wouldn't be an evolutionary advantage for a T-rex hen to be notably larger than a T-rex drake (or whatever the sexes are called), since anything that might attack their nest is getting squashed anyways.