Baileys will give you the measurements for a lot of the pistons. Here's the 046 meteor.
•Stihl 046, MS 460
•Replaces Part Number 1128 030 2009
•Bore 51.96mm
•Height 39.25mm
•Head Type Flat
•Piston Pin 12mm x 35mm x 7mm
•Ring 52mm x 1.2mm (Caber F-Cast GH.F)
•Compression/Pin Top to top of Piston 15.5mm
•Compression/Pin Center to top of Piston 21.5mm
064 specs
•Stihl 064, MS 640
•Replaces Part Number 1122 030 2001
•Bore 51.96mm
•Height 43mm
•Head Type Flat
•Piston Pin 12mm x 33mm x 8mm
•Rings 52mm x 1.5mm (Caber F-Cast GH.F)
•Compression/Pin Top to top of Piston 17mm
•Compression/Pin Center to top of Piston 23mm