ballpark idea

I would like to do a project like RQ Riley's urbatrike. The general idea is an advance motor belt drive. I would like to use li batteries but I dont understand battery terminology.
Now I know there are a lot of factors involved, 40 mph is average speed and I have a 50 mi round trip to work. My big question is how big of a battery pack am I seting myself up for. http://www.rqriley.com/u-trike.htm thanks elect3