They're all the same size for a set. So if you get a .020 (50mm) oversize, they're all the same size, all 8 of them.
The factory used select fit pistons which were about .0002" - .0003" difference in size (not even a whole thou .001"). So you'd see some cylinders (not just 7 & 8) with difference sized pistons.
When you rebuild a 6.5, you're almost always going to be boring the cylinders.
You measure your pistons about 1" from the bottom of the skirt, which is where the piston is the largest. Then you'll add how much clearance you want to run, refer to the service manual for piston to bore specifications.
Typically .0035 to .004" overall bore clearance.
On cylinders 7 & 8 (I do #6 as well on non-spray blocks) You add .0005" of clearance (yes half a thou).
So, cylinders 1-6 would be .004"
and cylinder 7 & 8 would be .0045"
1982 C10 SCSWB 6.5TD, mods too extensive to list. (13.69 1/4 mile @94.6 MPH) RACE TRUCK
1982 C10 SCSWB 6.2NA, 2.73 700
1986 C10 SCLWB 6.2TD 3.73 700
1989 V20 SUB 6.2NA, 3.73 400
1994 G20 VAN 6.5NA, 3.42 60E
1994 K20LD ECSWB 6.5TD, 3.42 80E
1995 K20 SUB 6.5TD, Wrecked, ran into by stupid teen.
1995 C3500HD DRW 6.5TD, 12' Flatbed 5.13 80E
1995 C3500HD DRW 6.5TD, 18' Rollback Wrecker 4.63 80E
1994 C20HD ECLWB 6.5TD 3.73 80E Wifes Truck.
1995 C20LD ECSWB 6.5TD 3.73 80E
1995 K20LD SCLWB 6.5TD 3.73 80E
1996 K30 DRW 6.5TD 4.10 80E
1997 C10 Tahoe 2Door 2WD 5.7L to 6.5 Conversion Underway