They're pretty much the same size as those in the 1st and second gen Mazda 3 (both C1 cars) so not anything like a Mercedes S class, but not nearly as bad as a coupe either. I don't ever sit back there but I've had 2 or even 3 adults back there no problem. I expect the same room for the FoRS......well maybe a bit less because of the chunky Recaro front seats, but oh well!
me and 3 friends (all good sized guys) took my ST on a 4 hour road trip to VT just fine. i specifically asked if they were comfortable as that was the first time i really had big people back there. they had no complaints. leg room was tight behind me but im tall. plenty of room for children back there. like others said. it's not huge, but adequate.
With my '13 ST2 the problem is getting in/out is a bit tight, but there's actually a decent cavity in the center of the seat for some leg room. Mind you I'm 6'4". It can be done and it's not that bad. People complain less thanks to the 4 doors even though my GTI had more leg/shoulder room