I've been playing a while & I noticed that w/ each world a new deck was available (assuming you reached a particular level) that had more slots for spells... I'm now at a point where I have way more spells than I have room for...but so far (I've gotten as far as Marleybone) I've seen no decks that offer space for more than 30 spells... have I just not progressed far enough to get a bigger deck, or do they all max out at 30 & to use the more powerful spells I have to buy a new deck & include those cards (discarding some of the weaker ones that I still like using) and switch to it?
The decks do indeed get bigger as you progress to MooShu and Dragonspyre. You will probably find, however, that having a smaller deck will be more efficient in terms of playing. If you consider the probability of drawing any one spell in a large deck vs. a small deck, you will get the spells you need more often by having a smaller deck.