About a litre
6
Depending on how it is geared around 70 mph.
The best dirt bike for a would be a Honda CRF 100 for a male and a CRF 70 for a girl. Of course the real size of the bike depends on the size and experience of the rider.
With stock gearing around 50 mph
Honda xr/crf 100 would be better.
CRF is just a label used by Honda in classifing the type of bike. for example Honda has lots of crf's. 50, 70, 80, 100, 150, 230, 250, 450.
I think a little 100 would probably do. I would suggest getting the new 2008 Honda CRF100F.
It should fetch something like $3000 at least if it is in good condition.
Depending on its condition and looks, in running condition, anywhere from a 100 USD to 500 USD for a bike in mint condition, looking as new.
A dirt bike would cost about 100 dollars in MX vs. ATV untamed.
An XR650R honda will do triple digits if geared right. It has been said than on the beach and paved sections of the baja 1000 that this bike will do over 100 mph.