Yep, well, from euro car parts I had to measure up. If you take measurements, it's just the metal core you're interested in - don't worry about the surround. You might find it easier to measure with the front grille off.
Although having said that, GSF just quoted one price and didn't mention anything about measurements. They quoted about 85+vat if i remember correctly but at the time I thought ECP were selling them for 40
For the record the one they're sending me is 630mm x 320mm x 32mm (width, height, depth) although i think that can change depending on the year of the car, 8v or 16v and whether or not you've got aircon.
Good luck
Hopefully i'll have mine by friday - i only ordered the it on saturday. Oh, hang on, no, that's not very quick is it?!