The scale/zoom is like a magnifying glass. The words on the page do not change, but they do get larger so you can see them better. If you zoom out, you get more area and smaller print. If you zoom in, you get a larger image and less area.
For example, USGS topo maps used in our custom prints were originally produced at a 1:24,000-scale (in most cases). You can change the scale to cover more area (by selecting a lower-scale/higher-number such as 1:30,000 or 1:50,000) or less area (by selection a higher-scale/lower-number such as 1:10,000).
Selecting a larger paper size will allow you to cover the same are at a more zoomed in scale. If you are going to need to select 1:35,000 or 1:50,000 scale in order to cover the entire area, you may decide to order two maps at a more zoomed in scale.