I believe you used a low level utility like dd or similar to flash an image onto a partition.
You need to run fsck on that partition, then run resize2fs to expand the ext2/3/4 partition.
Let's say your partition is /dev/block/mmcblk1p1
You would do (in a terminal):
adb reboot recovery
# umount /dev/block/mmcblk1p1
# e2fsck /dev/block/mmcblk1p1
# resize2fs /dev/block/mmcblk1p1
After that, your partition will get expanded to whatever size you made it.
It takes a while so be patient. The larger the partition, the longer it will take.
Nook Color running CM7 Mirage, no gapps, Nook Tweaks app @ 1.3GHz (+500MHz past stock 0.8GHz)
Nook Tablet running CM10.1.3, no gapps, BexusNv4 kernel @ 1.3GHz (+300MHz x2 past stock 1GHz)
SGS2 T-Mobile (unlocked) running HellyBean 4.2.2, no gapps, @ 1.72GHz (+~200MHz x2 past stock 1.51GHz)