They are rebuilding a bridge in my town. I noticed they are using I-beams. Would this work for an anvil if I can get a piece of it and how long of piece would i need?

The flanges on "I" beams in my opinion a just a tad thin for good anvil material. Now if you turn them on end so that you have the full depth of the beam and put a thick chunk of steel on that end, say 2" or more they make a decent support for that, then you have the entire weight of the "I" beam under your hammer blows otherwise not hardly worth the effort to make an anvil out of the flanges unless it one of those monster "I" beams with 2" thick flanges, now that is a whole nother matter. :D

What you are looking for is solid metal under the hammer face; now an I beam---even quite a large I beam has mostly air under it. Compact chunks of steel make much better and less noisy anvils.

However sometimes bridges make use of quite thick steel plates at the ends (depending on design) and a chuk of one of them would be quite nice to have in the shop.