We will never know what that .css file was since you have deleted it, I was suggesting looking at the physical location of the file 'in' the temporary internet files location, for size and the files properties (right click menu). That may have provided some information, but we will never know.
You don't know for sure what was in the .css file and I doubt it was 5GB, what you didn't mention was what type of scan you did, thorough with archives or what. Any scan with archives enabled will return a greater 'scanned' size than the size on disk as avast counts the size of files unpacked from archives to be scanned. In theory since avast didn't/couldn't scan this file, it shouldn't be included in the total size of scanned data (15GB).
It can be investigated here first, but you have to remember a file that can't be scanned is just that, a file that can't be scanned. Where this file was reported as a file that can't (or rather won't be scanned) because avast believes it 'might' be a decompression bomb, boy do I hate that expression as it strikes fear into users. It is not something that is new and has got into the wild, it is just a suspicion by avast that this file if unpacked could be very large, why it is very large isn't stated other than that daft term decompression bomb.
When fear strikes users act in a way that could result in the deletion of files that are 'suspect' only. One of the most common files, a .cab (windows cabinet file) could be very large and might also be reported in this way and deletion could have an impact on your system at some point in the future. So care has to be taken and it investigated fully, there really should be no way an .css file could be this big (your 5GB speculation), it is being downloaded over the internet and it would take ages even on a broadband connection and an eternity on dial-up.