[Germán Carrillo's](http://gis.stackexchange.com/users/4972/germ%c3%a1n-carrillo) answer is the most convenient. A possible alternative (if you're stuck with QGIS 2.8) could be to:

- Use a **string** which allows the user to enter the units they want to use:

		##units=string mm

		if units == 'mm':
		    # Do something
		elif units == 'cm':
		    # Do something
		elif units == 'm':
		    # Do something

 [![Text units][1]][1]

---
- Or use three boolean checkboxes:
 
		##Millimeter=boolean
		##Centimeter=boolean
		##Meter=boolean

		if Millimeter == True:
		    # Do something
		elif Centimeter == True:
		    # Do something
		elif Meter == True:
		    # Do something

 [![Checkbox units][2]][2]


  [1]: https://i.sstatic.net/tkW1x.png
  [2]: https://i.sstatic.net/Wn8fZ.png