I will be heating apx 1500 sq ft...if there less water will I get a cleaner burn ?
No,
You will not get a cleaner burn with a small boiler simply because
it will be cycling on and off, opening and closing the automatic damper
creating a lot of smoke which is lost energy as the smoke is not consumed
and simply wasted.
The more water you heat the greater the thermal mass
and the hotter the firebox which will aid in combustion and
draft and the reduction of smoke equals saved energy.
You can add thermal storage like the new horizons water storage tanks which
are 490 gallons each and have a huge heat sink in the water volume and the
tanks are insulated.
The other thing to remember is you can have small very hot fire going
and keep a huge mass of water at temperature simply by throwing wood
in the boiler occasionally to keep the fire going.
The more water you have the more useable energy you have from the thermal mass.
The forced draft aids in combustion and when the boiler gets up to temperature the
easier it is to maintain the temperature simply because you have huge amount
of water that will not cool down quickly.
It only takes small amount of combustion to bring the large volume of water
back up to temeprature.