It's big, very big compare to the 300, but the airflow is incredible.
I've got 1 CM 690 and 2 antec 900. The CM690 is a little bigger than my 900.
Cable management is very good, excellent airflow (you can add a few more fans if you want), a lot of room if you need to play inside the case and a few options/goodies on the case.
The only thing I hate on the case is it feel cheap, the cover is made of cheap-like plastic and the power button feels very cheap.
I've worked with 2 of the CM690 and on the second tower the front plastic cover broke quickly.
But overall the case is good. My antec 900 feels very solid compare to the CM690.
If you don't have to move the tower then the CM690 should be OK. Anyway you don't want to move the CM690 because it's heavy and big.