measure dropout to fit chain tug
There are two dimensions that matter:


Axle diameter. 10mm is the standard, but you'll also find models designed for 9.5mm (3/8") and 14mm axles on BMX bikes.
Dropout thickness. This is the thickness of the metal plate comprising the dropout when looked at end-on. The tensioner cups the ends of the dropout, so must be large enough to fit over them. Steel dropouts are typically 5mm thick, while aluminum frames can have 8mm thick dropouts.


In both cases, you can use a caliper to determine the dimensions precisely, but a simple measuring tape or ruler should be sufficient.