I am Tim , a different expert.I will be happy to assist you with the information you are looking for.
To begin with, the only way to be sure unit is sized properly is to do a load calculation on the home.THis is known as a Manual J Load Calculation.It is a program where you enter all of the building construction factors such as square footage of home, construction type such as wood frame or brick,how much attic insulation, wall insulation. whether on a slab or up off the ground on peers. the square footage of glass windows and type of windows facing each direction, whether there are covered porches shading windows,number of occupants(figuring 2 per bedroom),number of fireplaces if any, number and type of exterior doors,ceiling types,geographic location and other such factors.For example a 1500 square foot well insulated home with double pane windows on a slab with 8 foot ceilings and very few windows facing north with 2 bedrooms in Northern Michigan might require an 80,000 btu unit to properly heat the home where as the same1500 square home in South Alabama might only require 50,000 btus to heat properly.
This takes about an hour and a half to two hours to do a manaual J by entering all the construction factors.Most contractors will run a manual J for about 200.00 and add it to the price of the installation.Many states are requiring the manual J in order for permit to be issued and inspected.This protects the homeowner and guarantees unit is properly sized.
If you can provide me a few details like where you live, amount of glass facing north ,how well insulated and whether on slab or piers I can give you a closer idea.