Not sure exactly what an 8GB Nexus 7 should give you but that def doesn't sound right, have you checked the device settings?
Press Menu, either from the App tray or pull down notifications and press the three sliders. Scroll and select storage.
Failing that download and install a file explorer from Google Play, there are plenty like Astro, ES or Solid Explorer and check the internal storage manually.
It could be that when you flashed the custom ROM the flash went wrong and changed the partitions?