Here's an excerpt from Produce, Publish, Publicize with tips for your posters:
If you intend to hold several author events, I do encourage you to get a large poster of your headshot and book cover to advertise your presence. There are a few places online where you can order a large poster for around $15.00. To get a nice large poster you need to be prepared with a very large image with 600 or more dpi (dots per inch). Your cover designer will be able to provide this file for you.
Major publishers print out 2’x 3’ or larger posters for the bookstores to display for each book signing depending on the budget. It probably won’t be practical for an author doing their own publicity to create this size of poster for every signing as the odds are he or she won’t make their money back from books sold at the event.
You can get creative with poster design so that you change a portion of your poster as many times as needed. For example, you can design a poster with a blank space. In that blank space, attach a chalk board or white board to write each event’s details. The downside to that is a prankster can erase the details and add their own message. Another option is to add a sleeve to the blank spot where you can slip a piece of paper with the event’s details. Any sort of page protector would do, but for durability, try a photo album sleeve. You can purchase those as loose pages at shops like Hobby Lobby. Your local Walmart might have them as well.
A simple 8 1/2” x 11” sheet is sufficient for getting the information out for most book signings. A smaller sign is perfect for your venue to post on their door or bulletin board. Smaller signs are also less costly to produce so you can modify the poster if you want to post them in other places as well as your signing venue.
The poster should always start with a visual of you so people know who they are looking for. Next, what you are advertising. Next, when. These are the most vital parts of the poster. Without this information, this poster is worthless to the reader. The middle part of the poster is advertising filler. It gives more information to the reader as to the topic and relevance. The last line should always be an invitation. Invite the reader to the event in a way to interest them.