How do I know what size my photo is and how can you change the size?

To see the Image size you will scroll under the Image window to Image size

See below.

For placing images in other programs like Quark Express, Pagemaker or InDesign, optimally you want your photo(s) to be at 100% up to 110% at 300 dpi.

Photos that come off camera cards come in various dpi ranging from 72 dpi up to 480 dpi (or more).

The sample image size below 72 pixels at 24" x 15"

Step 1.
Open your photo. Open the Image Size. Write down or remember what the top pixels are. This example was 1728 pixels and the resolution was 72.

Step 2.
Next change the resolution to 300 and then change the pixels at the top back to 1728 and you will have your new size.

So if your image needs to be bigger than 5.76" then you can bump the width up to 6" or 6.5". Bumping the resolution up makes Photoshop extrapolate what pixels to fill in to make up for the increased size. Sometimes it looks really bad when increased too much.