What deck size is ideal?
Yes, and the limit may surprise you: it's 60 cards, which is exactly the minimum number of cards in a deck.

Why is this so? It's very simple: some cards are more powerful than others, but (with a few exceptions such as Relentless Rats) you can only include a limited number of copies of them in your deck. Generally speaking, you want to maximize the chance you draw your most powerful card(s), and apart from including as many copies as possible (four), the only other thing you can do about this is including as few other cards as possible. That means running 60 cards, and not a single one more.

Of course, this is a general rule, and there are some exceptions (e.g. if you build a deck around Battle of Wits you'll want somewhat more than 200 cards, but not too much more) but they are few and far between. In Limited tournaments (e.g. booster drafts) the minimum deck size is 40 cards, but again the ideal deck size is equal to the minimum size, 40.