Unable to change package footprint solder pad size in eagle
Looking at it your footprint uses a circle drawn on the tstop layer which is why you see diameter setting when you look at info.

The pad itself is actually an SMD pad, which does not have a diameter setting. Instead it has two key properties:


SMD Size which changes the overall size of the copper pad
roundness which will be set to 100% (i.e. circle - 0% would be square)


So to change the size, leave the roundness set at 100%, and change the SMD size setting, making sure to keep both the width and length of the pad the same.