If you’re looking strictly for file size, not dimension, play with file > save for web (Alt + Shift + Ctrl + S).
select the "2-up" tab at the top left of the screen. You will see the original file at the left, and the ‘optimized’ image at the right. I like to zoom in one level and then play with the settings until the file looks almost as good as the original. If you’re new to this tool, the main settings you’ll want to play with are Colors (if saving as a .gif) and quality (if saving as a .jpg).
You’ll see the file sizes for the original and the optimized version below the preview image. This is a fast way to cut down on file size.
My Photoshop file is too big and I need to reduce its size. How that can be done?