Wedding invitations can be any size, but the most common sizes are 5x7 inches and 4.25x5.5 inches. The thickness and weight of the card will depend on the type of cardstock you use. Cardstock ranges in thickness from about 80 lb. to 110 lb. and heavier. A heavier weight cardstock will feel more substantial and formal, while a lighter weight cardstock may feel more casual.
It's important to consider the size and weight of the envelopes as well. Make sure the envelopes are large enough to fit the invitations and any additional inserts, such as response cards and maps, and that they are sturdy enough to hold the weight of the invitations.
Ultimately, the size, thickness, and weight of your wedding invitations should be based on your personal style and the formality of your wedding. You may want to consider ordering samples of different cardstocks to help you make your decision.