How to make a dirt bike street legal in New York State?

So i've been reading a lot of the posts about turning a "off-road use only" dirt bike into a street legal "enduro" style bike. How is this done? Some people say its not possible anymore. Others say it is possible if you register it in another state and transfer the plates. If so how would this work? Also what are the physical requirement of s dirt bike to be street legal (such as parts).
Any help, tips, and advice is appreciated.

Thanks.
Also - is a 250 too small?
What size would be appropriate for a 20 year old male 6ft 160lbs.