Mac storage basics · Finder tools for finding space
How do I sort files by size in Finder?
Short answer: In list view click the Size column header (add it by Control-clicking the headers). Turn on View → Show View Options → Calculate all sizes so folders sort by size too.
Switch the window to list view (View → as List, or Command-2) and click the Size column header; click it again to reverse the order. If there's no Size column, Control-click any column header and tick Size.
Folders show two dashes instead of a size until you turn on View → Show View Options → Calculate all sizes. With that on, folders sort by their real size too, though big folders can take a moment to measure.
To sort a whole disk rather than one folder, use a Finder search instead: find large files on your Mac.