If you have access to registry and if you really need file greater than 250kb, you can change value in Registry Key.
For an x32 system:
Create the Key:
HKLM\Software\Microsoft\InetStp\Configuration\
Add the DWORD value:
MaxWebConfigFileSizeInKB
For an x64 system:
Create the Key:
HKLM\Software\Wow6432Node\Microsoft\InetStp\Configuration\
Add the DWORD Value:
MaxWebConfigFileSizeInKB
Be sure to reboot or restart the application pool. This answer to a related question may also inspire.
Note: If there are not the key, you can just create it. how to create a key on regedit
Other alternative is to split the .config files. http://forums.asp.net/t/1961321.aspx?Split+web+config+rewrite+rules+in+separate+file