To create a different sized world in Terraria:

1. Go to Clanforge -> Manage -> Edit Profile (In the left sidebar) -> Configs tab -> serverconfig.txt

2. Find the following lines: 

#Creates a new world if none is found

#World size is specified by: 1 (small) , 2 (medium) , and 3 (large)

autocreate=3

Currently it is set on "3" (large) but you can change the "3" to which ever size you'd like. 

3. Next you have to get the server to generate a new world. To do this you have to find these lines:

#Load a world and automatically start the server.

world =$$ConfigPath$$\Worlds\world1.wld

And change where it says "world1" to any new name. 

4. Click Save and Apply and your server should restart with a new map.