Question: What should my seal size be if my shaft diameter is 25mm / Or your shaft diameter?
Answer: The same size as the shaft diameter.
This is because the manufacturers of the seals make allowance so that the seals will be a snug fit on a shaft.
E.g. If you have a 25mm shaft then the seal's bore you require will be 25mm as would be marked on the seal
In reality if you actually measured the bore of a new seal it will be nearer 24.2mm, this is the allowance that the manufacturers build into the seal.