Here is an electric treadmill that fits under your desk to give you a workout. The Goyouth 2 in 1 Under Desk Electric Treadmill comes with a LED display and wireless speaker. It has a 2.25HP motor and non-slip running belt. It has a speed range of 0.5 to 7 mph.
This treadmill has a treadmill running belt of 41.34″ x 15.75″. You can find out more about it here.