Resizing posters

Is there a way to make posters size to a new image? The G image (green/blue is 640x640), and the blue/white one is well... 1920x1080. I know how to do it manually with the right shift, but it's not entirely accurate. Is there a way to make them snap to what the actual image is, and not to what was lasted used for sizing? These images were not resized, rather the last poster I did was for a chess piece that I did resize. I'm thinking there is something really simple that I am over looking at getting the correct size right the first time.

Thank you.

I figured out my own solution.

1920x1080 is 16:9 scale. (16 across, 9 down)
Turn on Grid to resize.
Unfortunately 9 doesn't play well with the Grid, as it will center itself every time no matter the size of the grid.
The grid loves 16, as you can see. Easily alignment with the grid.

Perhaps better clarification is in order. Allow the poster to be reset to default image scale, and have a check box to lock scale when resizing as an option.
Do I have this posted in the wrong section?