the maximum bag space is 45 slots. You buy the upgrades from the guys in the other cells when you put items from digging on the table to the cell to the guard, who will give you a short window to buy upgrades. to get items a lot cheaper you buy items in the warehouse once you obtain the key from the coffin. im using WeMod free version and modded the game to give me heaps of cash of toilet paper, it still unlocks the achievements. you can look at the mirror to see your stats.