The sizes of FWs are variable to say the least - I would say that Pillow Torques length is comparable with Rips height at the back. There are no size limits in the FRA build rules just as long as it stays within weight (currently 12kg) - of course when you get the essentials in there (batteries, motors etc), the rest is yours to play with. Although clean air may save you some potential damage by putting vitals out of reach, you have to weigh up (scuse the pun) the pros and cons of the type and thickness of armour you can put on there - it may mean that the dimensions may have to change to accommodate it all.
BTW PT is currently 500 x 330 x 100 mm (50 x 33 x 10 cm)