Just picked one up today. 86 chevette. I believe its the same one they sell on Danger Den and the same one that Hoot used for the waterblock shoot out. Both Hoot and DodgeViper have made great homemade shrouds for the cores and 120mm fans.
I believe the exact measurements are 155mm wide by 185 mm tall (including inlet/outlet height)
Edit: Hey JokerF15, Just realized I ordered a Gemini for your site last night. Any estimates on when I will see this thing. I realize it will have to pass thru customs to get here to Canada.
I have heard that the heater cores from Chevy Chevettes were basiclly what has been used by some of the water cooling kit makers. Ive tracked down some of the information on them and by my understanding all that it is, is a heater core - Chevy Chevettes - Years '74 - 84. You can go to your local car shop and get one, with some you might have to place the order for one. You will also have to put in a few barbed connections which is actually very easy to do. I took JBWeld and placed the barbs on the inside of the connections (after cutting off a small portion of the heater core inlet/outlet) and theyve stuck pretty well so far. I havent as of yet had the chance to test it up, but it looks pretty solid.
I was checking at AutoZone the other night and found the core from like a 93 Geo Metro would work well too. I believe it was just about the right size to put 4 80mm fans on it with a minimum of waste if you wanted to do something like that.