i saw the antec 900 and i thought it was quite small. as for if it will fit, then technically yes. the antec 900 is 20.5cm wide, and the tuniq tower is 15.5cm tall. so yes. then again, you might have a really retardedly layed out motherboard...so what kind of mobo do you have?
In it you will see aa few people post pics of their 900's and I do believe in the last set of pics (post #21 which if I linked properly it will actually put you there) is a Tuniq Tower in there. So yeah it fits.