Chainsaw high quality T-Wrench 3/4" (19mm) for spark plugs and 11/16" (17mm) for bar nuts. 

Fits
Stihl 050, 051, 075 (older), 076, TS350, TS510, TS360 and TS760

Dolmar 122 and 144

Echo 1001

And more

The barrel length is 2-3/16" and the shaft length is 6-11/16"