Can you provide a little more information? What size (in pixel dimensions) is your original, and what pixel dimension are you saving down to?
Are you using Save for Web?
As soon as you wrote you need a 72dpi image its clear you don't know what you need. It seen you want or need a image file no larger then 6,000 bytes. However a 6K file can contain an image with a vast amount of pixels or a small amount of pixels. Image file size is dominated by Pixel data. However Pixel data is usually compressed. Compression size particular Jpeg compression varies all over the place based on the quality setting you use. Low quality setting produce smaller size files then high quality setting. If what you want is a image the will fit in a web page. Most likely you will need to reduce the numbers of pixels you have for you image. DPI is meaningless for web images. For displays do not display image at the image dpi setting. Display only display the pixel size they are set up for. So you web image will be displayed low resolution like 84 to 140 dpi on desktop display an will be displayed at 100 to 300+ DPI on phones, tablets and laptops.
You do not want to Print a 72DPI for printers do print the pixel size specified by the Image DPI setting....
I am submitting an image for publication online. The site requires a file no larger than 300 K. My existing HR image is 144 Res; 4 W x 4.44 H inches; 1.4 Mg. When I change the settings to pixels, the units change to “percent”?? Frankly, I don’t have any choice about pixel dimensions. I must create a file that is no more than 300K. No matter how small I make it, when I save it as a JPEG, the final size is about 600K. I am doing a save as to produce a JPEG file. Not using Save for Web.
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You can optimize your file size to your specifications by using the Save for the Web Optimize Menu which is located in the upper right-hand corner of the Save for the Web user interface. The optimized Menu offers the option to optimize to file size and desired files size in Kilobytes.
File |>> Save for the Web.