The larger the number after the colon, the less detail you will see in the map. 1:5,000,000 might be considered a relatively small scale map.
The smaller the number after the colon, the more detail you will see in the map. 1:25,000 might be considered a relatively large scale map.
In general, it is a good idea to be flexible in how you look for a map. For instance, if you are looking for a map of Seoul, Korea, also look under Korea, East Asia, Asia, and World, assuming that the scale is suitable.
Major maps series held in H-L Library. (The Maps Collection is on the lower level, past Government Documents on the way to the stacks in Hubbard Hall.)
Maine Transportation Maps
U.S. Government Maps