There probably are ways of doing it through maemo, but this is probably the easiest way of doing it. It's basically solution 8 from the wiki

1. Clear unwanted files in MyDocs first, you will need 3GB free to shrink partition, plus maybe 4.5GB if you backup OptFS and RootFS later on.
2. Backup settings etc using maemo Backup utility.
3. Backup MyDocs.
4. Install BackupMenu.
5. Restart n900 with keyboard open to activate BackupMenu.
6. If you get errors on the n900 about missing libacl.so.1 and libattr.so.1.1.0 files, just press enter - they aren't required any more.
7. Use BackupMenu to backup Optfs - in fact why not backup both RootFS and OptFS like it recommends.
8. Press w on n900 to activate USB Mass Storage.
9. Connect n900 to PC with USB cable.
10. Copy OptFS & RootFS backup files to PC from Nokia N900/systemBackups folder.
11. Start GParted on PC.
12. Press w to restart Mass Storage mode on n900
13. On GParted select the emmc device from the drop down menu on the top right of the program. (See where mouse cursor is on the image) It is the 29.82 GiB device. Check that the right partitions are there -> ~23GB Nokia N900 / ~5GB ext3 in your circumstances.
14. Right click on partitions and resize to required size.
15. Click on Apply to resize.
16. On completion close GParted, press q on device to restart and close keyboard to prevent BackupMenu from activating.