First off, you are smart to take advantage of the radiation in your house and distribute the heat where you want it. A stoker boiler will be a great upgrade for your home.
Hot water boilers are rated by the sq. feet of radiation they can serve, and with a btu/hr figure. A KA-6 has a "recommended load" of 675 square feet of hot water radiation, and 122,000 btu's per hour of Net output. If you want to know how to measure the radiators, read this guide: http://www.columbiaheatingsupply.com/pa ... 0Guide.pdf
If you were building a new house, a heat loss calculation would be done to see how many feet of radiation you need in each room, and how large of a boiler you need to heat the house. You have an older home with unknown R values and air exchange rates, so without spending a lot of $ on some fancy thermal analysis and blower door test...you will have to estimate those things if you want to do a heat loss calculation.
I have done it both ways, and when it comes to an older house...I like to do a "short hand" version of a heatloss calculation to get a general idea of the total load, and also do an analysis of the existing system (rating, run-time, and fuel consumption). In your case, you don't know how much fuel the existing system used, or how well it worked...so all you can do is see if the radiation is capable of meeting or exceeding the heat loss, and then picking a boiler that can meet that load.
The reason I recommend doing a heat loss AND measuring the radiation is that many older houses were over-radiated. If the heat-loss works out to 100k btus/hr at 0 degrees, but there is enough radiation in the system to spec a 200k btu/hr boiler...you may decide to pick a boiler that can comfortably run 150k btus/hr (just an example).
I recommend you take the time to measure the radiators in each room, and do a heat loss calculation using free software. Slant-fin has a free program that is easy to use...maybe someone can post a link to it.
Now, about the Keystoker KA-6...do you have access to buck size coal? A friend of mine has been struggling with a Ka-6 for a week, it has been running hard to heat his house and creating clinkers in the stoker that sometimes stop the flow of coal. I gave him some buck to mix in with the rice, and now it runs great with no clinkers. If your heat load comes in on the high side of the KA-6 capability, I would plan on running buck or getting a bigger boiler.