It's (quite) easy to check the print size.
Here's a little mini-guide to scaling things in Photoshop that might help you:

If you want to change only the dpi (to 300 dpi) but keep the pixel size:
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles" and "constrain proportions" are checked. "Resample image" should NOT be checked.
Type "300" in the resolution box.
You now see the correct print size!
Click OK.


If you want to scale the image and increase pixel size:
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles", "constrain proportions" AND "Resample image" are checked.
Type the new pixel size click OK.
(Be careful when you increase the number of pixels of a picture - you'll always loose quality.)

Does it make sense?