David,
I used the letter size pdf file for mine. It does actually come out to approximately half the size of a medium size bald eagle. Also make sure your printer is set on 'no scaling'. If it is set on 'print to paper size' or 'print within margins' or any other such settings it will distort the parts.
I don't really know about making it bigger as my thing is to make things smaller. Although if I was to want to make a bigger one I would take it to a print center to have it printed on larger size card stock as my printer does not print bigger. If you are wanting to make a full size eagle you would want them to print it at 200%.
Sorry I can't be of more help,
Kathie