measure dropout to fit chain tug I want to buy a chain tensioner, like the one in the first picture in this answer.

I did some research and it looks like they have different size, according to what they call "dropout size". In fact, 5mm and 8mm seem to be the two most popular sizes.

What does this measure refer to? How should I measure my frame to be sure that I buy the right 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.