A 748 range duke has a 520 chain the 99* range having the 525.
Size doesnt allways matter as much as the quality of the components materials.

The majority of racers inc GP use the 520 erv, a narrower but better quality material.

Oh generally, you pays your money... but sometimes can get ripped off!

In GP250 most people use Regina 520 chain-non 'O' ring! The 'O' rings may seal the grease in for road bikes but they also cause drag and you don't need drag on a 250. I lube the chain on my TZ with WD40-again no drag. Oh yeh, nearly forgot, Regina chain is expensive as well.;)

John