I'm trying to print a map using the whole size of paper... possible?
At the bottom of the map window (left there are a few icons.
One is for the entire extents of the data. the other is for the paper layout.
When you select that the default printer is configured with the default paper size and is normally filled to the printable edges of the paper.
You should right click outside of the data frame (which is the zoomed extents of the data) and select page and print setup.
In the dialog select your hp 900 and the select the setup or properties button next to the printer.
Select your paper size and in the page layout tab (each driver is different) there may be a setting to automatically rotate paper to conserve paper.
When finished with the settings, ok to accept changes and then left click inside the data frame to select it.
Right click inside the selected frame and select distibute> fit to margins.
You should have a paper with your map filling the page. It would be appropriate now to change the zoom or the scale of your map on the paper.
Something about your question sounded as though you wanted a landscape orientation. If so in the page and print setup dialog when you set the properties on the printer you should select a landscape oriented paper size and then in the last dialog you can selecct the landscape setting.