Sometimes it is important to know how large a file is, particularly when you are working with pictures, video or music files (often large files).
Windows: right-click and choose "Properties."
Mac: right-click or ctrl-click and choose "Get Info" or simply select the file and use CMD-I.
Sizes are in KB or MB (=1000KB). Emailing anything larger than 3MB is not recommended, and 500KB or less is good for inserting images into documents or web pages.