You can review your file sizes in your File Manager. The far-right column ("Size") displays the size in KB of each file.
File Manager Size Column
To check the size of a file on your computer, you can open its properties. If you are using a PC running windows, right-click on a file, select "Properties" from the menu that appears, and look for the file size. If you're using a Macintosh, you can open a file's properties by single-clicking on the file to highlight it, then opening the File menu and choosing "Info."