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
Captured German WWII Maps
Army Map Service (AMS) maps
Army/Defense Mapping Agency (DMA) maps