46MB. However, that image includes three separate complete copies of all the compiled code (4.5-6MB each), so that it simultaneously boots on 32-bit BIOS, 32-bit EFI, and 64-bit EFI. It also includes the complete source code (24MB) needed to reproduce the image, to make it self-contained and redistributable without needing to have a separate distribution of source. And it includes .py files for all the BITS .pyc files (other than the Python standard library), to simplify debugging and hacking. A quick check suggests that you could pretty easily build an 8.4MB image, if you cared about the size.
> Is it modular, e.g. if you don't need networking[1] how hard is it to not include networking?
No. We haven't needed a smaller image, so we haven't worked on that. Typically, we boot it from a USB drive or hard drive, so storage space isn't a concern.
Networking, though, consists almost exclusively of Python code, and the compiled .pyc files for that don't take up much space; not a good target to start with if you wanted to reduce size.
[1] I'm using networking as an example package, not knowing what other semi-optional packages could be used as an example.