In any photoeditor there'll be a point in the menu where it says "Change Size" or just "Size" or something similar.
If you find it there'll be a box where you change the width and the height. A landscape could be around 900 pixels since most screens are set to about 1100 pixels wide or more.
I like to do my standing photos at about 700 pixel width, that way you don't have to scroll down to see most of the pic.
Also it makes it better for speed and up-/downloadtime if you reduce the quality of the picture. Most people won't notice if you change the quality of the picture to about 60%.
Oh, and do remember to use "Save As" when working with pictures. If you just press "Save" you might lose the original picture.
HTH
Kyrre