Jami,
I agree with ueli lüthi and his way of approaching mast dimensions. My rough way of estimating a boats max righting moment is to multiply its displacement with an estimated righting lever, which on your boat should not be more than 1/5 of the beam
Mr= Disp x beam /5 = 1542kg x 2.21m / 5= 681kpm
I would aim for a an aluminium (lower) mast with about twize the yield strength of the estimated righting moment. I try to keep the wall thickness no more than 4% of the diameter.
I suggest you look up what tube is available between 12 and 15 cm, and what alloy (temper) is being used.
The mast of my 2.15ton Ingeborg is 15 x 0.5cm, of 6082-T6 alloy, and that is slightly oversize for your lighter boat, me thinks (yield strength, My=2037kpm, weight 6.38kg/m).
Arne