In case you were wondering, your wizards all automatically get an additional 50 item space to their personal banks, raising the maximum from 100 slots to 150. As well, you can purchase an elixir from the crown shop for an additional 50 slots to your backpack. This only applies to one of your wizards, though.
The elixir costs 2500 crowns AND it is repeatable (you can continue to increase your maximum by 50 item slots for every 2500 crown purchase). 2500 crowns isn't cheap, but if you're constantly bumping into the maximum because you're farming dungeons or just forgetting, this could be a big help.