In all honesty, project audio, wave files, and video files take up the most space. Using "Per-Project Audio Folders" will contain project audio in subdirectories of the project itself. My F: drive is where I store these, and the paths are (Preferences->File->Folder Locations):
Project Files: F:\Cakewalk Projects
Wave Files: F:\Cakewalk Content\Audio Library
Video Files: F:\ (these are largest and I want them very visible till I move them elsewhere)
Looking at my list, I think I specified "Cakewalk Projects" and "Cakewalk Content" to the F drive during the X3 installation. ***Depending on the size of your SSD, the below may be totally unnecessary***My SSD is not huge (256GB), and before I realized that video was eating up my SSD, I also moved the Dimension Pro Multisamples folder. This folder is 6GB and was not necessary to move (it must be done manually) once I realized I had 40+ GB of video to get rid of. Just in case you do want to move this one day, it requires a registry edit to
HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\dp
for the "Mutlisamples Folder" entry to the path you want (mine is F:\Cakewalk Content\Dimension Pro), and manually moving the Multisamples folder from the SSD to that new location.