How do I design the circuit for baseboard heaters?
Sizing conductors and breakers
Almost all wire you can buy will be rated at 600 volts, so you won't have to worry about voltage when sizing wires.  What really matters is current.  Your wires will be sized to carry enough current for the load, and your breaker will be sized based on the wire used.
When installing electrical devices, it's always a good idea to check the manufacturers installation instructions (PDF).  In this case, it has this to say...



As the table shows, you can connect up to 24 amperes worth of heaters to a single circuit.  However, if you do this, you'll have to use 10 AWG conductors and a 30 amp breaker. If you only want to use 14 AWg, you can only put 12 amperes worth of heaters on a circuit.
Number of conductors
Since these are 240 volt heaters, you'll only need two conductors (plus a grounding conductor). If you're going to use nonmetallic sheathed cable (Type NM), you'll need AWG/2 with ground, not AWG/3 with ground cable.