As koraks says, screens don't know about inches, only pixels.
In Photoshop, you can find the [Image Size] dialog under Photoshop's [Image] menu heading. Change the units from Inches to Pixels, if necessary.
If your goal is to reduce the image resolution (pixel dimensions) for the purpose of posting online, then - with the Resample box checked - just enter the pixel dimensions you want. For online viewing the longest edge should probably be something between 1000-1600 pixels. If the Width and Height are locked, enter one value and the other will automatically change to keep the same proportions. This will also reduce the file size (in MB), but I suggest you work with pixels and let the MB take care of themselves. If a greater reduction in file size is needed, lower the JPEG "quality" at the time of export.
Keep in mind that the Resample process results in a lower resolution image, so export the resized image for posting but do not Save the resampled image as a replacement for the original image unless you are certain you will never want to use that image for some other purpose in the future - like printing it. If you "Save As" the resampled image will be saved as a copy, and you will still have the original; but if you "Save" then the downsized version replaces the original.
Make sense?