I'm having a hard time printing a map within QGIS. For instance I did a 18x24 inch map printed on a 18x24 inch map. But the printed a map on a 9x12 inch printed on a 24x36 piece of paper. Why is it hard to print a big/wall map out in QGIS? Is there a step I'm missing?
Sounds like you are not setting your composer size correctly. When printing, the QGIS Composer has 2 different places you need to set the page size, and they DO NOT affect the other.
1) Page Setup: I'm guessing you set the page size using Composer > Page Setup. This sets page size being sent to the printer.
2) Composition tab: In order to change the page size of the document workspace, change the "Page Size". This is most easily done with the "Presets" pull-down menu.
To demonstrate why this might be helpful, imagine this scenario. I am designing a large poster, but I just want to print a copy in my office to see how it would look. I might set my Composition page size to ANSI E (34x44) but set my Page Setup to ANSI A (8.5x11) and send my poster to my letter sized printer for review.
For me personally, I prefer to export my composer map to a PDF file instead of sending directly to the printer. That way I can confirm my page size and/or use Adobe Acrobat to scale up/down automatically as needed, matching the page size in the printer.