Hello,
I have one boxed DT 125 1st o/s piston,pin,rings and circlips. 1st o/s is 0.25mm
As Mervin said, the pistons should be stamped to show the o/s, (if standard size they will have a part number stamed on them instaed) if not then you need an internal micrometer to measure the bore. You also need to evaluate the situation to determine what size you need as the cylinder could be worn or scored.
Ideally you need to take this to somebody who knows what they are doing as you cannot explain these things over the internet.