OK what gauge wire do I need to power up A 240 volt 10.3 amp baseboard heater with 220?
NEC Section 3:10-16 calls for #12 which is rated for 20 amps which is almost twice as much as the load you are going to put on it. I'd say that's pretty much like old men wearing suspenders with a belt to make sure their pants don't fall down.
You need a 12-gauge wire so as to power up this heater. You can talk to an electrician to be able to get more details in this regard. Read http://www.ehow.com/facts_7729475_gauge-220volt-hot-water-heater.html to learn more.