I am building a garage with storage trusses. The garage is 24' x24' I was wondering what size center support beam I need to use?

First off... check with your local builders.

In my area (south louisiana) a 3 x 15 or 5 x 15 Glu-Lam beam could be used.

Not sure what you mean by "storage truss".

http://i819.photobucket.com/albums/zz111/lacogada/24x24_zps67621e67.jpg

A manufactured truss is self supporting and does not need a beam.

What are storage truss?

Chuck