Welcome to the Forums randomstu. Give ES File Explorer a try. It's a free app in the Play Store. Then get Clean Master Cleaner also free from the Play Store. Under Junk Files in that program there is a tab called Residual Files which lists big files left behind. You'll probably see the ROMS you downloaded there and can delete them if ES Explorer doesn't do the job. Clean Master is also very useful in clearing out your cache files which can build up pretty fast. I use it at least once a week to keep as much space free on my device as possible.
"You silly twisted boy, you" - Hercules Grytpype-Thynne