Hi Dave -- this is likely one of those issues that's easier to resolve for a specific case than for all cases. What dimensions do you regularly print? On what size paper? How much space do you need between images? How much space do you need between the outermost images and the paper's edge?
The dialog has some decision-making built into it that can be frustrating. It appears that it will allow you to change image size as long as that size fits with the current row and column settings. If it doesn't, you have to change the row and column settings first. When you change those, you images size is changed, and the number of images per page is fixed. You can return the number of images per page setting to "as many as possible" by clearing the field and tabbing out of it.
HTH.