I think some form of dimensional increase is inevitable, taking the evolution from the z3, through to the z4 e85 and on to the e89 into account. Model sectors are just becoming really bloated. Just look at the official dimensions of the current e89 in comparison, starting from the z3.
I drove a 2 series coupe recently and thought it compared very closely with my old 3 series e36 coupe.
However my e46 m3 felt comparable to my e36 dimensionally, certainly from inside the cabin, so i don't think it's exponential.
I hope they do a good job on this one and haven't allowed Toyota to compromise the chassis or driving dynamics in any way. I understand they will be sharing chassis and engine/drivetrains.....