Most such printers have a feed tray of some kind which is 8.5x11, but they also have a feed slot which is 8.5 inches wide. You may find that you can reconfigure the printer to print on 8.5x14 paper and "manual feed". The printer will then wait for you to insert the 8.5x14 paper before printing it.
For double sided printing, you wait until the first side is printed and then re-insert the paper with the other side up to print the second side.
You may have a printer that only printes 8.5x11, but it doesn't cost the manufacturer much to handle the situation as I have described, so they tend to include those capabilities.
I'll get some paper stock and give it a try. Thanks.