My cat is nearly a year and has just had kittens - this was also unplanned.
It is not so much the carrying the kittens but feeding them will be a dreadful struggle for her in my opinion - make sure she gets lots of high quality food - remember she should still be on kitten food herself. In my opinion cats are no-where near full size at 6 months even if the growth rate slows after then. My girl still looks a bit kitten like and I don't expect her to be quite her full size until 18mths and think they gradually increase in size until around 3yrs.

Emily