If the servos are attached to the side of the fuse box on belly, the std 9g Servo cable should not need any extension lead.
Any was a 15cm lead should be enough.
Other wise just tie a thread on the plug of the servo and measure what length you would need to reach the Rx.
Actually many reason for not able to check myself
1. the kit is packed ( I am in korea and flying back to india)
2. There are no servos installed
3. I dont know the placement of the receiver
4. I am not sure of the wire routing.
so it is impossible for me to check Myself. so looking out a help from a jet experienced flyer