Can I search for folders of a certain size (say more than xxx mb) I am cleaning up a bit. I know how to search for files of a certain size (see below). Is there a way to search for folders above a certain size? Shell commands are ok, but gui is preferred.

To search for files above a certain size:



If you don't see the "file size" option, select "other", it will be there.

then:



This is great for files, but it does not include directories/folders. Also, please don't delete files that you do not know what they are for. Some are low level system files and can cause problems.
I recommend using a disk space analyzer application for this purpose, specifically because of the concerns you bring up.

Some options include DaisyDisk (Free to try, Full Version US$9.99) or  GrandPerspective (free), OmniDiskSweeper (free)