G'day Achilles,
Ok first off, you can use a 5 cell, 6 volt pack with that RX, sometimes some servos are not suitable for 6 volts.
A 60 inch plane is relatively small, & an 1100 Mah pack will do nicely, unless you are flying with digital servos & flying 3D.
The capacity of the pack, is derived from the capacity of the individual cells, because they are connected in series, so a pack with 1100Mah cells is still rated at 1100Mah, the capacity does not add on.
You will need to charge new packs at 1/10 of its capacity, so an 1100Mah pack should be charged at 110ma. see site, http://members.aol.com/davthacker/balancecharging.htm

I personally still use nicads, because I have had trouble getting NiMh cells that will supply reasonable current, but NiMh packs are good if you can get the good ones.
It is a good idea to replace you batteries about every 3 years, so anything over 1100Mah, up 2000Mah will be good for your installation, remember the larger capacity, means more weight.