When I took my long arming class, we used a solid fabric and drew on "blocks" to work in. This is definitely what I would recommend for learning - you can see your stitching, you don't waste time piecing something that will have really random quilting, and solids are generally less expensive.
Depending on the length of the class, whether you have your own machine or share, and the techniques covered, a piece 1.5-2 yards by WOF is probably sufficient. A few people have suggested using a dark thread so you can see your stitching easily, but I would go with a lighter shade than your fabric - in my experience, it looks better, which is nice for a first attempt (even though I think I'm a fairly good quilter, my first long arming was rather shaky). As long as it isn't identical to the fabric,you will be able to see it well enough.
Good luck! I think you will love it!