The manufacturer often publishes the length of their antennas.

If you need to get another piece of wire to splice on, the ideal fix is to replace the whole antenna rather than splicing in the middle.

If you're not really good at soldering in small places, the best option might be to send it to a repair center such as [link=http://radiosouthrc.com/]Radio South[/link] and have them do it. In addition to ensuring that the right wire is used and that the soldering is done right, they'll be able to do a quick tuning to make sure the receiver is operating at peak performance. The total cost of doing this will be relatively low and you'll have the peace of mind of knowing the right wire was used, that it was installed properly, and that the receiver is tuned to the new antenna length.