I think you would want size the nozzle to the input of your furnace. If you under size the nozzle your furnace will not come up to its eff. Your stack is the right size for the furnace and nozzle. Most oil is forth about 140,000 btu per gal. What ever nozzle you use you can times it by 140 and that gives you the btu input

Some burners are labeled with a range of nozzle sizes for that burner. A nozzle nearer the smallest size adequate for your premises and also within the permissible range for that burner should produce lowest heating cost. A smaller nozzle needs less air for proper combustion. Excess combustion air lowers efficiency. Smaller nozzles cause the furnace to run longer with each cycle. Each oil furnace startup is the most inefficient moments of operation, therefore a smaller nozzle means fewer starts per hour and fewer starts means greater efficiency. With a new furnace, if the literature specifies the nozzle type, cone type, angle, use a permanent marker and write that on the furnace. Often service people just slap in whatever nozzle they have and even if the next service person is conscientious, they will install the same (wrong) nozzle. Today, many HVAC service people have little training on servicing oil burners. Get a book by Burkhardt on oil burners and learn about them.

Here is a link that might be useful: Book

Most new oil systems sold today have been thoroughly tested to determine the correct nozzle size, spray type and spray angle. Bear in mind that all nozzles are rated in gallons per hour with a pump pressure of 100psi. However, almost no burners today are shipped with the pump set at that pressure. Usually you'll see anywhere from 125 to 160 or so depending on which burner it is. Beckett and Carlin usually don't go over about 140 while Riello may be as high as 180. Therefore the actual nozzle may be smaller to achieve the correct firing rate. Example, .75GPH nozzle at 130psi gives a .85GPH firing rate.

With many of the newer furnaces the fan is controlled by an electronic panel that starts the fan after so many seconds of the start of a cycle. It assumes a certain heat exchanger temperature based on the specified firing rate. A lower firing rate could very well lead to a lower exchanger temp which will create condensation inside. Condensation on steel = rust.

Bottom line, read the manual and use what's specified.