Deck size in Limited format

As a general rule, you almost always want to play exactly 40 cards in Limited because it maximizes your odds of drawing your best cards, since in Limited your 41st card will be by definition worse than the first 40. Larger decks are less consistent.

The only one exception to this rule that you should be aware of is when playing against an opponent trying to mill you. How to defend against this scenario? Sideboard in every card that is remotely playable in your colors as well as enough basic lands to keep your deck stabile. For instance, if you have six playable cards in your colors in your sideboard, bring them in along with four additional basic lands. So, your starting library will be 50 cards now and it buys you some time to kill the opponent. The reason you don't want to bring in 100 basic lands is because then you will only keep drawing land cards. In spite of playing against mill deck it usually still has some creatures to deal you some incidental damage, so 100 basic are not a good idea. In many cases those just extra few cards in your library will be the difference between a win and a loss. Drawing a weaker card that you boarded in will only rarely cost you a game against a mill strategy. Remember, that your liberary is your main resource here, not life total. However, mill strategies are not popular in Limited.