Not sure on the beam size, but you will need some pretty decent supports for the beam, and decent footings under them. I have a 12 ft wide 2 ton portable gantry crane (professionally designed and manufactured, not home made) that has an 8" beam, don't recall which one right off.
Here is info on Wide flange beams (W series, also commonly called H beams due to their shape, and they have flat flanges).
http://www.engineersedge.com/standard_material/Steel_ibeam_properties.htm
Here is a chart of S beam or Standard beam sizes and weights. These are what people commonly think of when they think of an I beam, again due to its shape. It has tapered flanges that allow a trolley to self center.
http://www.engineersedge.com/standard_material/Steel_ibeam_s_properties.htm
You have to select a trolley for the type of beam you will be using, and S beams are more suited for the purpose you have in mind.
If you happen to use an S10-35 beam, then it is ten inches tall and weighs 35 lbs per foot, or in your case, a 700 lb beam. At least you are not trying to span some huge distance. Just be prepared to spend as much on the support posts as you will for the entire rest of the project.
Charles
