In answer to your first question, I noticed the same thing; the icons you are using will maintain their aspect ratio. The way to get them to fit is make sure the ratio is correct. In your case you said you made the icon in the dialog 50 by 50, so make sure your .jpg is exactly as wide as it is tall, and that should fix it.
size of your dialog in centimeters? iunno. I think the options are just dbu or pixel width, neither one will tell you how many cm it is.