well there are a lot of methods to increase the optfs i have mine 8gb
the method i use is hexing the emmc.bin (file) before then reflash n900

to do this :
1-download and install any hex-editor (i use neo)
2-grab the emmc.bin (file)
3-open it with the hex-editor
4-search for pattern (partition)
5-you will see a bit below number : they are (2048)
6-change them to any size you want until 9999
7-reflash your n900 with emmc.bin (file)

for me i changed them to 8192 so i got 8gb of optfs (to install applications)

btw you can use backupemnu to backup your rootfs and optfs then restoring them after you reflash your n900

and i think there is a downloadable 8gb emmc image if you want google it