How far can a 2x4 or maybe 2x6 support over a 10' span?
I do not know how your side supports are constructed or how you plan to keep it from racking side to side. 

Three 2" x 6" set on there side so they are like floor joists, I.E. not laying flat.

One front, one middle one back, use Simpson joist hangers to attach to side support rim joist. 1/2" Plywood decking on top of that to create a shelf. 

If you want even more weight capacity overkill make it four joists. 

How much weight will it hold ?  There is an online sag calculator you can use https://www.woodbin.com/calcs/sagulator/

It is a garage shelf, if it sags a little over time it is not the end of the world. When it does, Turn your  joists over in the joist hangers so the crown is on top and get a few more years out of them. Or use 2"x 8".