The best Decks are around 40 or 50 cards. If you have a Deck that is a lot bigger than that, and can not find a way to make it smaller, here is a trick you can use.
First, pick the 5 most important cards in your Deck. The cards that you have to keep.
Then take the rest of the cards. Look at the first 2 cards. Pick 1 to keep in your Deck, and put the other card back in your collection. Then look at the next 2 cards, keep 1 for the Deck, and put the other in your collection. Keep doing this with all the cards.
This will cut your Deck size in half. If it is still too big, do this again. It it is too small, then put in enough cards from your collection to bring it up to 40 cards. Now you have a lean and mean Deck!