There's a reason that decks in MtG are usually 60 cards: that's a good size for people to handle (plus, it's easy to get deck holders sized for 60 card decks, largely due to the popularity of MtG). So I don't recommend that you make a typical stack much bigger than that.
With a physical representation of mana (as opposed to just a number on a sheet of paper), it's much easier to implement systems where you regain mana over time instead of all at once. In your place, I would take this opportunity to get rid of Vancian magic; you might be more attached to that system than I am, and obviously this is your game that you're designing. Give people a way to regain mana during the day, and it becomes a per-encounter resource. Additionally, the more often you can regain mana, the less total mana you need, and so the smaller your stack has to be. This makes it easier to handle, play with, and shuffle, and makes it more predictable, but abilities that put unwanted mana into your stack are more effective.
I'm not very clear on the rules for regaining mana in places with aspects. If my maximum stack size is 60, and my stack contains 40 blank mana when I go to sleep in the underworld, do I just get 20 death mana (enough to fill my stack), or is all of my blank mana replaced with death, so that my stack now contains 60 death mana?
What happens in places with multiple aspects (two or more minor, a major and a minor, or a major and two or more minor)? The kindest thing you could do would be to let the player choose what kind of mana they get: up to one for each minor aspect, and the rest comes from the major aspect (or blank mana if there is no major aspect).