Full details are on this page:
http://www.firstclassmotorcycles.com.au/partFinder/fiche/yamaha/1994/tt250r-ttr250/rear-arm
Item number #25
or item #24 in http://www.partzilla.com/parts/search/Yamaha/Motorcycle/2000/TTR250+-+TTR250MC/REAR+ARM/parts.html
Thanks,
Arthur
learning to ride on 1993 TTR 250
Ex 1997 and 1998 Honda XR 250
Looks like I found the size: 20x27x25