[Dabhand16]

When I 'save as' a jpeg, there is a dialogue box that appears just before the image is saved where you can set the level of compression. Depending on the image, I can set between 8 out of 12 up to the least compression of 12. I average about 9 or 10 for most images.

If you have a 'save for web' option in the file menu, you go to another window with various options, and you can see the size of the saved file displayed.

When I re-size to 72 dpi and 800 pixels, when I OK it the image shrinks to the size of a thumbnail, and it is this image that I save.

To keep reasonable quality you should be aiming at 7 -8 in the jpeg compression (or above)