I'd say either 40- or 60-card.

But if you're running a 60-card, make sure that it has a lot of duplicates. Lot of 3 and 4 of everything.

There's more to making a simple deck than just size. The main thing is simplicity. Here are some constraints I'd suggest. This I call "New World Order on Steroids"

1. Make only 1- or 2-color decks
2. Limit yourself to creatures that are vanilla, french vanilla, simple ETB effects or simple "dies" effects.
3. Have 6-8 vanilla creatures per deck
4. Don't have any non-evergreen keywords, and maybe even avoid the more obtuse evergreen ones
5. Focus on only a handful of evergreen keywords per deck
6. No planeswalkers
7. Only basic lands or the very simplest duals (Salt Marsh/Submerged Boneyard cycle)
8. No equipment, and no more than two of Auras, non-Aura enchantments, or non-creature, non-Equipment artifacts
9. Limit yourself to the most basic of sorceries and instants, like Murder.
10. Go heavy on draw/discard and life gain/life loss. Everybody's played games where you draw and discard cards, while it is imperative that you understand life gain and loss by the end of your very first damage.
11. Go light on things that require a lot of tracking. For example, it's OK to have a sorcery that involves the graveyard, but avoid Lhurgoyfs. Avoid mill or any alt-win-cons.
12. Build decks with a single, easy-to-understand, overarching strategy. For example, a white/black life gain/life loss deck. Or a white/blue air control deck. Or a white/red weenie deck. Or a Relentless Rats deck.

You might want to check out this thread where I created 60-card decks based on those constraints. http://forum.nogoblinsallowed.com/viewtopic.php?f=16&t=16947&start=40 Some are more complex than others. I believe the easiest to learn.

I think the previous posts beg two questions:

1) How far along before intro players learn the stack?

2) How far along before intro players need to learn the real value of cards?