Transformer before or after switch?
The switch should be prior to the transformer for several reasons.
First this kills power to the transformer; if the lamps were only disconnected, even with no load, transformers waste power. This use of power usually increases with age and later it may start buzzing as the core delaminates. The power usage in this case is from eddy currents.
Second, with the switch turning off power to the primary, if you need to work on it the circuit is dead, you did not say if this is a hard wired application or a plug in.
The size of the wire would be based on the current the lamps draw. Since it is about .83 amps each or 1.66 amps, if it is not a long distance 18awg wire would probably be fine, but if it is a long distance I would verify the voltage drop.
As Harper mentioned halogens, I would bet you could find replacement LED’s that will last longer and draw ~1/3 the current.