Hi tapimjay,
There's no way at the moment (that I'm aware off) to explicitly change the size of the icons within Fusion. There have been a large number of requests, via the Ideastation, to increase the working area of the screen and various people have sited decreasing icon size as possible solutions.
From what I understand from reading the forums, the design team are working on the screen real estate and part of that ongoing process they have rolled out a preview ribbon toolbar layout which, though not finished, can be seen via the preferences > preview and UI Preview.
I tried it and I really like where it's going. There are a few issues which need to be worked out e.g. I couldn't find the parameter Icon which I often use so I reverted back to the original and eagerly look forward to the next iteration.
Matt