Prior to printing, determine the desired final output size needed. ArcMap, similar to many other programs, sets the default page size to 8.5 x 11 inches. To customize the page settings for poster-size printing, first click on File and the Page and Print Setup.
The Print Setup dialog box will appear momentarily. Browse through the list of available printers to select GISPlot, Plot03, Plot36, or Plot42 (Plot36 and 42 are for faculty and staff only).
Click on the Properties button located next to the printer name. Choose the desired orientation and then click on Advanced.
Click on the down arrow for the Paper Size menu. Scroll down the list until you see Post Script Custom Page Size. Choose this option and a new window will open.
Enter your desired output size in the appropriate section. Remember to leave enough room for the plotter margins. Click OK until you have exited all of the page setup menus and have returned to the ArcMap desktop. Switch to Layout View.
Once in Layout View, you may need to adjust the viewable area of your map to fit your custom page size. Simply click and drag your map outline to increase or decrease its size. Due not that this will change your map scale, and it should be corrected once you've adjusted the viewable area. Additionally, the Layout View parameters should be the shape and size you defined in the preceding steps and serve as a "Print Preview." Before printing, add any items necessary such as legends, scale bars, etc...
When ready, click on File -> Print. You could be in Layout View when printing.
The image in the bottom right of the Print Dialog box affords a simple preview of your print job. It does not include data from your project (it uses a stock image), but does show you how the map is arranged on your page settings. Be sure these settings are to your liking before printing. Click OK to print.
ArcMap is able to adjust certain types of data to whatever print size is desired. Vector data (point, line and polygon) can be resized with little to no image quality loss. However, raster data (DEMs, etc...) will become distorted if enlarged too much. This should be taken into account when determining your final output size. The same holds true for any images inserted into your final layout, such as inlays or other image documents.