How do I control the image size in dividing a poster for printing?

I am trying to tile together several USGS maps to make a room sized topographic map of my region.  The USGS offers pdf's that are 33x34".  I want to print them from my home printer on 8.5x11" as posters to maintain some of the topographic details.  But I've found that some of the maps have different white page boundaries, which doesn't change the map scale as a single image (it's own poster fits right back together), but it does affect the continuity of scale between the different pdf's.  The "Tile Scale" feature when adjusting the cut lines in the "Poster" option within the "Print" dialogue box, only allows for adjusting the percentage.  This means that there are not actual dimensions to correlate between different maps.