I made a New Years' card for you to download and print. The card is an A6 size (105 x 148mm or 4.13 x 5.86 inches) and fits a C6 envelope (114 x 162 mm or 4.5 x 6.4 inches), which can be trimmed out a letter-sized piece of cardstock.
The card has bleeds. and is set with crop marks and fold marks to indicate where the card is to be folded. If you give the file to a printer, ask them to score and trim the cards for you. It's usually just one or two dollars extra for a stack.