the status bar at the bottom of the finder window shows the # of files and the free space avaiable on the harddrive. Is there a way to make the finder show the total size of the contents of a folder? either in the status bar while you are in a folder, or in the main pain when a list of folders are shown.
In Finder List View, you can have the Finder calculate the size of Folders, however, it is very disk (I/O) intensive
Finder -> View -> Show View Options -> Calculate all sizes
If you have a solid state drive, this might not be too expensive (in time and power), but on a rotating disk, it eats your battery power, makes your disk busy when you want it to do other things, and takes time.
Enabling it to maybe check for large folder trees can be useful, but for that I would just download and use OmniDiskSweeper (free)