Changing file size of an image in PhotoShop

Original file size was 27.8mg width 3872 pixels height 2592 pixels

By going to Image > Image size I changed the dimensions to 400 pixels wide and a height of 268 pixels.

Change the resize measurements to suit your needs.

File size is still 314.1kb.
By saving for web devices

We can then have a final image of 17.9kb having the quality setting left at 60% and JPG.

And the final image

Hope you find this helpful.