I do not believe you can (a) enlarge the window of the application or (b) use the app to find flight distances.

For a very close approximation on flight distances, I use WebFlyer's "MileMarker" at:
http://www.webflyer.com/milemarker/milemarker.htm

It is usually very close. For instance, it estimates SFO-BOS as 2700 miles, whereas United says it is 2704.

Hope this helps,
-b-