Are there any rules regarding the maximum number of cars in a deck?

Is it necessary that the decks of both players have the same number of cards?

Check page 18 of the rulebook. Under Deckbuilding, you must have a minimum of 50 cards in the deck, and maximum of 100 cards and no more than 3 copies of any single card. You cannot mix Destruction and Order races and no you and your opponent do not have to have the same number of cards, as long as you both abide by the rules above.

Kevin.

The above post is correct for tournements. You and your friends though can have as large a deck as you want in my opinion, at least thats what we do and find it very fun!