Stihl 038 p/c size

[Harmon]

Howdy! I am tearing into what appears to be an 038 Farm Boss, according to the tag on the shroud. It runs, but just ok and appears to have been tinkered on previously. Am in the process of replacing the rubber parts, fuel line and two of the av bushings were shot and out of curiosity pulled the cylinder off. Understand that there were three options, 48, 50, and 52 mm - my question is that the measurement of the cylinder bore, or the piston? much thanks

 

[L34]

Bore.
48 standard,50 super,52 magnum.