1. The original RailKing f3 locos were really short. When MTH reproduced the Lionel postwar F3 in the RailKing line, the shorty F3 was renamed "rugged rails". So searching for rugged rails F3 will get you a nicely condensed F3 loco for your railroad. There are some B units on eBay right now, but no A units that I could see.
2. Also, Williams produced ALCo FA locos that were copies of the O-27 postwar Lionel ALCo FA units that are quite undersized. You have to be careful looking for this one because some were produced with only one motor. Williams made the ALCo FA in both scale size and O-27/postwar size, so be aware of that also. The small one is called the FA-2 in Williams parlance. There are several FA-2 locos on eBay right now.
Either the Williams FA-2 or the MTH rugged rails F3 should do what you want on your railroad.
3. A third choice is the RED Chuggington loco (the blue one is bigger). It is a caricature of an F3. Some of the major train companies have them on sale for around $90 or so.
Some of our sponsors/ vendors should have these locos.