I have a Canon Imageclass color laser printer, MF620C, windows 10. It has a "multipurpose tray" where you can manually feed the paper. I am using Adobe Indesign and my document size is 8 x 24 inches. However the printer won't print anything outside the standard size of 8.5 x 14 even if I go to custom settings, it won't let me register a different size in printing preferences at all. Any help appreciated. Thanks.
Well you have your answer, 24 is larger than 14. You can try printing it twice, if the paper feeds through OK, just rotate the page and rotate the other print job on it's axis if whatever you are using to print allows that. You may end up with a gap in the middle though so you may need to experiment with the print size to make them flush. Or print two pages and tape/glue/etc.. them together.
Or are you trying to just print that size document on the standard size paper and shrink it? That is easy to do, just set the scaling to lower and it will fit.