As previously mentioned, the load on the loft is what will tell you how big your beam needs to be. It is a bit of a complex problem, as you will have support from the 3 walls and the load will be distributed across the floor. The next part of the problem is what material you want to use. As previously mentioned, steel and engineered beams would require less cross sectional area than regular wood.
Another option is to hang a post from the ceiling at the center. This keeps it open underneath, and allows the possibility of using shorter beams. That may require beefing up the ceiling attachment point.
As for getting a beam that is 25' long, if you use 2x12s or 2x10s, you have to attach them together as Hickey says, but you simply stagger the ends. Cut one at 6', one at 12' and one at 9', attach those together, then lay your next 12' piece and start attaching that. Don't know if that is clear or not, let me know and I'll try to be more clear.
