I'm not selling this one anymore so you're welcome to use it for personal use only. It's 5x7. If you will need envelopes, cards and pockets is the cheapest place I've been able to find for specific quantities. Happy Haunting!
If you have a Mac and use Pages and Template Chooser, they have a TON of awesome templates you can use for Halloween invites and flyers.
Otherwise, I'd suggest playing around in Word, Adobe, or Paint. You can download free fonts online to find something spooky that works for your theme. This year my invite is really, really simple, it's just a flyer. It's a still shot close-up of Frankenstein's Monster [Boris Karloff version] that I tinted green and then overlayed classic Halloween fonts with the information.