Nope, you can raid 0, with different spec/size 2 or more drives. Although the stripe set can only have a maximum size of the smallest harddrive.
Although hardware manufactures will always recommend that you use 2 of the same identical drives to get the very best performance.
From your post though, it just looks like your needing more space, and not really a raid0, which would offer no redundancy, so if 1 drive fails you loose everything.