It will automatically fit to the image that you drag to the cell.
If you want to cut the image you need to do this in Develop module first (hit R).
I don't see the need for this "button" ... please explain in more details if I did not understand your issue.
I use the lightroom print module to print several images on a paper roll. When i add cells of certain measures i can simply drag the pictures in the cells and go on. To optimize the space sometimes i need to rotate the cells. Yes, i can drag an image in a cell for example AxB and then click rotate on the cell to let it be BxA. But this is already when you drag the image in the cell.
Sometimes i just enter the width and height in the opposite way and i have to click again on the custom cell windows and rewrite it.
I need the button inside the custom cell window.