I have installed WoL, size 14 GB, when I buy HotS I'm afraid that might grow up to 30 GB, which is too much. Since I'm not going to play campaign, is there an option to install online multiplayer only?
Nope, unfortunately you cannot pick and choose. However, HotS was less than 5GB for me, you should be fine.
EDIT: I did digital download, IDK if a physical copy is bigger/smaller.