It may be that it's not possible because gparted didn't provide a GUI control for it, but the mke2fs program certainly does have the -i option. You may have to resort to formatting it ext3 manually from the CLI instead of letting gparted do that part. You can see the inode size by using the program dumpe2fs like this:
Code:
dumpe2fs -h /dev/sda1
or whatever your partition is called.