is like asking what is the post card size? some post cards are in 3R, some are 4R, some even 5R, it really depends on what the designers or publishers want.
anyway, 4R prints is the most cheapest in term of cost to paper area, so you can design your thank you card utilize the 4R paper, be it cut in to 2 pieces, 3 pieces or 4 pieces in the end.