rousta,
Most design engineers find a maximum heat load for thier buildings and/or process and then go out and purchase a boiler(s) that will meet the maximum need
Firetube boiler designs are the result of both design/heat transfer analysis as well as experience.... This is a very mature area of engineering. Many boiler design firms have settledon standard designs and appropriate sizes that will meet the market demand.
Is your question really about "steam system" design ? or sizing maximum boiler demand for a prospective installation ?
If so, there are software packages available for these kinds of investigations..
MJC