Help Determining Boiler Size!

So, I have had something like 8 estimates for a new oil boiler at this point. Only one took measurements for a load calc and, although that won him great favor in my mind, it took him nearly two weeks to get me an estimate and then the estimate didn't even say what size boiler he would recommend and, to date, he has not answered any of my follow up questions. But I digress...

One guy told me he could determine the boiler size if I got the average K factor from the oil company, which I did. Actually, I got two: the prior owner's oil company said 3.93, and my company from last year said 4.35.

So, what does that tell me about boiler size? (and, yes, I already know I should probably insulate my house better.) First and foremost I need to know whether to get 3, 4, or 5 section boiler, but also how many BTUs? Planning to get a Buderus G115 with 40 gallon indirect, and logamtic control.

Additional info: House is about 2400 square feet (that's a real estate measurment, not necessarily interior), thick stone walls, built in 1940.

Thanks.