453 MB on my install. It's such a useless app that I used it once, configured everything, and then verified that the same mouse worked fine with libratbag on linux -- it does (I have it on a KVM box between two different machines). Next time I'll just configure it on linux. Their hardware is good -- but the software is awful.
However, I'd prefer native, non-Electron app, that doesn't take 350 MB of space, just to configure my mouse (G Hub).