OK, I took some measurements with one of the TEC cupholders I have in stock...
Maximum diameter at the base of the cup is 66mm.
35mm up from the base of the cup, the maximum diameter is approximately 67.5mm.
These measurements are approximate, and of course if you're using a styrofoam or paper fast food cup there is plenty of "give" in the cup to fit into the TEC cupholder; This is certainly true of most S/M/L size cups, however XL size cups and Super Duper Huge 1 Gallon Big Gulps aren't going to fit.
The TEC cupholder is designed to work with one of the BMW insulated travel mugs. There are several styles available; See the below picture. I prefer the one on the far left. I can supply these mugs, too (and will be adding them to the website soon).