First, to be able to compare these two things, you have to either find the radius of Mercury or find the diameter of the moon. I choose finding the diameter of the moon because it seems easier.
Diameter=radius*2
Diameter=1736.5*2=3473
The moon has a diameter of 3473 km.
Since Mercury's diameter, 4878km, is greater than the Moon's diameter, 3473km, Mercury is larger.
Hope this helps!
