I don't think that the size of the cube (within reason) affects speed.
If the cube feels comfortable to you, and you think that you can get as fast as you want to with it, then keep using it. If it feels too big, then get a different, smaller one. If you don't think that you will use the cube forever, then I think that you should get a standard size (5.7 x 5.7 x 5.7) cube. This is because you'll get a different, smaller one later. Adjusting to the smaller size will probably be hard to do.