A new SPL has been made available for the G1 which almost doubles the size of the main system partition, dramatically improving the overall performance of the device!
I have installed the SPL (available here) on my G1, and using ADB, these are my partition sizes:
You can also view your current partition sizes by installing 'Terminal Emulator' from the Android Market, launching it and then typing 'df'. I'd be interested to hear what people's default sizes are (especially if you're on a Magic!) The SPL is installed on rooted devices with an appropriate recovery partition by downloading, renaming to update.zip then flashing via the recovery partition. Your main ROM also needs to be reflashed afterwards, but there is no data loss.
Some devices seem to take issue with this update! I flashed 2 G1s. 1 flashed fine, 1 is dead. The problem seems to be related to the board version. If you turn off your G1, then turn it back on while holding down 'camera', the top line shows your board version. It will say DREA100 then either PVT or something else like DVT2-1. My PVT device flashed fine, my DVT2-1 device is toast. This is not 100% confirmed as the cause though, so flash at your own risk!