The size of each partition is up to you. The 1st/fat32 partition is what Android uses for "external SD" (for photos, media, CWM/TWRP backups, etc.) and the 2nd/ext4 partition is what the hack uses for space (copy of the internal /data plus whatever extra apps you install since using the hack).
Once you're rooted, the next step is to install TWRP. The necessary info should be in the 1st post. Expand the "spoilers" if you need details about a particular step. I don't normally give step-by-step instructions because everyone's phone is in a different state. Read the first post and ask if there's anything specific you don't understand.
As for backup, start by backing up apps and app data in case something goes wrong. Since you're rooted, you can use Titanium Backup. Lots of guides online. Alternatively, Helium Backup is another option that's simpler. Once you've successfully installed TWRP, you can boot into it (but don't use the button combo from a powered off state to enter recovery on this phone) and make full system backups. Once you've made backups, copy files on the SD including the backup files to your computer so that SD card troubles won't destroy your backups. With the ability to restore the system back to a previous state, go ahead and install the hack. Good luck.