Doug, the SI cart ledge, that the smoker actually sits on, is 21". I have a Harbor Freight cart that I used with the analog #3. It sits 33" high. It was a little high, for the stock 3, but absolutely too high for the 3D (you can't read the display). Personally, I wouldn't go higher than about 24-26". You want to be able to look down on the top of the smoker.
The table Peter referenced, is 26" tall....perfect!