photo contest measurement/Adobe Photoshop

hi eveyone,
I would like to enter a photo contest, but I don't know how to make sure I am sending in the right measurements for the photos to qualify. I have Adobe Photoshop Elements 12, but am still new to it. I am assuming there is some way to fix the photos or check to see if it's those measurements through Adobe, but how?

The measurements they are looking for are:
1200 pixels on their longest side (what does that mean "longest side")?
Images can't be larger than 1 MB.
Winners need maximum-quality JPEG, 10 inches on the longest side at 300 dpi of their images for publication in the magazines."

any advice is appreciated, thanks :)