The nice thing about oversizing electric heat is that there's no real detrimental effect on the equipment. It will just cycle off a bit more quickly. You can do a heat loss claculation if you want to, and indeed it may be required. One watt per cubic foot for insulation values in accordance with Zone 11 (IRC/IBC) will do the trick. 1.5 watts per cubic foot is a little nicer for commercial buildings that might feature concrete slabs and uninsulated overhead doors. Not sure what the occupancy is exactly, but I have a feeling that you're going to end up using 15 KW of heat by some means, since that's a more standard size closer to your calculated requirement. Either three 5kw units heaters, or an air handler with a 15kw heat pack.
