A planet like Earth is like a big rock except it's molten underneath but that molten stuff is still rock. Planets like ours are extremely dense obviously because they're made of rock and not gas. Therefore they have a lot of mass and therefore a lot of gravity for their size.
So, if a rocky planet was too big the gravity would crush it because it doesn't have the nuclear fusion process to fight gravity like a star does. I just don't know how big is too big. I'm sure the size limit is much smaller than a gas giant like Jupiter.