wheel diameter for turbo trainer?

TRUFFLER 1 Posts: 23
edited February 2010 in MTB general
After some help please fokes, just unpacked my first turbo trainer and about to build it up but I'am unsure of my wheel diameter. Is it just a case of get the tape measure out and measure across the wheel or is there more to :?: The model of turbo trainer is a Tacx T1810 Seedmatic, the options are, as it is out of the box or it comes with a kit for 610-640mm or 690-720mm diameter wheels. Iam using my old mountain bike with it and using up all the old tyres I have knocking about, cheers for your time.