afaik its around 14MB but the large majority of that is the Bun runtime itself. at some point it will likely be possible to pick and choose parts of the runtime to include in the binary. thus the bundle size could get a lot smaller in the future
that said electrobun's author has published a bsdiff implementation in zig, and thats used for electrobun's updater. that means you download deltas, not the entire application bundle, every time you push an update to your users. and then it gets patched in-place.
this makes updates tiny, to the tune of a couple kB