Er, Ckclasse its a simple question -but the answer less so
Anyway lets try - the capacity of the cells is the time you get to sail, and the number of cells is the voltage
one lipo is 4.1 V (fully charged) and two are....... well done you at the back
I don't know exactly about spectrum receivers, but I have heard it said that they like 4.8V, not more
It
may be very happy with one Cell - most other Rxs work well on a single Lipo, and you will be sailing this fellow quite close to you. If it won't or if you don't want to try it then your only option is 2S Lipo (8.2V full). This is tooooooo much for the Rx and the servos- definitely
So you need a seperate BEC which can be bought or made (7805 and two capacitors) - this takes the volts and reduces them to 5V nominal for the Recceiver - which in turn feeds the servos
So that's it for the number of cells
The capacity - especially bearing in mind your weight problems
Forget the rudder servo and receiver as not eating much currrent.
How much will the drive motors take? Twould help if you told us the type and model, but we can assume an absolute max of say half an amp the pair
So if you chose 500Mah Lipos they would run at this level for an hour
So if my thinking is reasonably right I feel you should be looking for a 2S1P Lipo with capacity around 250, 350, or 400 mah capacity , or less if you need low weight
They are made and sold in capacities from 5mah to about 4000mah
Please let us know how you go on
andrew