It always seems really hard, but always stick to 60 cards - resist the temptation to use more! This may seem almost pedantic; after all, the chances of drawing any given card in a 61- or 62-card deck aren't that much lower. They are lower, though, and it's surprisingly noticeable. Here are a few reasons why it's good to be stern and cut it down:
-If you slip to 62, it's easy to slip to 65 once in a while, where the difference is bizarrely huge.
-Most of the time, when people include too many cards, they forget to up the land count to match, and end up getting mana-screwed a hell of a lot.
-Forcing yourself to cut cards will hone the final 60 to be the leanest and meanest of the lot.
-It maximises the chances of drawing one of your best cards, or the key cards of the deck; put another way, it's maximally efficient.
-It's good practice. This sounds like a stupid reason, but it isn't. Being strict with your 60-card limits gets you thinking about what the best cards for a given deck really are, and so increases your understanding of the game.
-Perhaps most subtly, it means that each card in your deck has a known chance of showing up. If all of your decks are the same size, you know that the 2 Righteous Blows in one deck have the same chance of being drawn as the 2 Shocks in another. Once you have a feel for how often a card shows up if you include one, two, three or four copies of it in your 60-card deck, you'll feel much more confident in deciding how many of a card you need.