Here is an exercise bike you can use under your desk to get a light workout. It has a whisper quiet pedaling mechanism and 8 adjustable resistance levels. The Flexispot V9U has a seat that can be adjusted from 29.6 to 37.2″. It can handle users up to 300 lbs.
The machine measures 37.6″ (L) x 22.8″ (W). You can find it here.