I don't have any issue with a size limit - but I wonder about the way it's implemented. Instead of enforcing a size limit and forcing the user to resize the image to be below the 5MB limit why don't you resize the image on upload? This can easily be done on the client with a few lines of javascript (keyword canvas) and is what's commonly used.
That'd be the most user-friendly solution as the user can use 'any' input and would allow GC to still enforce its limits.