Well, if I have to choose, I would say the extremely small. It is not necessarily because the large scale structure of the universe would be less interesting or fascinating, but we have discovered more aspects of the quantum realm; therefore, there are more interesting things to check out. This would probably be true of the macro realm if we had as much knowledge about it as we do the micro realm.
I would like to be small enough to check out the structure of space, and how it fits into the information available for the electromagnetic force to access. I would like to see how an electron can be two places at the same time, and how virtual particles pop into out out of existence, and from exactly where. I would like to see superstrings---up close and personal---and enter into their other spacial dimensions . . . wow! can you imagine what that would be like? Having 3 times the dimensions? And what is going on in those dimensions? I would like to see exactly what quarks are doing in protons, and how they are structured.
Oh, I could add a hundred more things to this list, but you get the idea.