That is really hard to say. The source code is massive - hundreds of megabytes. The image created is tiny. It all comes down to what drivers and options are included.
Most generic kernel package included in Ubuntu is around 121mb. But it includes a lot of modules - see:
http://packages.ubuntu.com/lucid/amd64/linux-image-2.6.32-21...
Very few of these will actually be running on your system.
I work on embedded Linux and once build your own kernel with just the drivers you need, the image is tiny. I know the exact number of the top of my head but it was either a few mb or maybe even less than 1 mb. We had 256 mb of total system flash and we got our entire Linux distro on there easily.
