Doing the apothecarist quest will unlock a potion for purchase that increases the size of both, you can get the quest by going to the board close to the entrance of the town.
I assume you're talking about the Ego potion? You must defeat the lich from the notice board called "Quest: Kill the Lich". It will ask you to talk to the Apothecarist. She is located in the shop to the left of the blacksmith (or behind you at the starting area.) with the sign "Potions" out front. Once you kill the lich a few new items will unlock in her shop, one such is the Ego Potion you're looking for. However, it does become irrelevant quickly if you just win battles in dungeons and even more so against strong (huge) enemies.