There are two questions that need to be addressed when selecting the appropriate size equipment for the application. First question is "What does the space need?", in other words, how many heating and cooling btu's? Answering this question correctly, is done by performing a load calculation. 

The second, and probably the most important question is "Will the current duct system handle that need?".  Just because the old equipment "Worked just fine" before, does not mean that the system was built correctly from the beginning. The new equipment may react differently (or horribly) if this is not addressed. 

For example, if you're intending to replace a 100k btu furnace, determine if 100k was even needed to begin with. A 100k btu furnace today will require approximately 1800-1900 CFM, to keep it in the sweet spot of the manufactures temperature rise range (45-50*F). So you investigate and you discover that your duct system is only capable (respectably) of providing about 1500-1600 CFM. Which would be more appropriate for an 80K btu furnace. You could put a 100k back in, but you'll most likely be called back by an angry client. 

It is important to know the Output btu of the equipment as well. The output of a 100k at 80% (approx 80,000) will be much different than the output of a 100k @ 95% (approx 95,000) This will impact duct system capabilities also due to the CFM requirements. The difference in 15,000 Btu's is basically 3 average sized bedrooms. 

This applies to A/C also, if you need 3 tons of air conditioning, will the duct system support it? (3tons @ 400cfm per ton= 1200CFM total). 

Keep in mind that load calculations or Btu requirements are based on worst case scenario. Heating at 0-5*F ODA and cooling at 95-100*F ODA for our area. And, duct systems are built and sized based on those requirements.  We spend very little time at these temps so looking into staged or modulating equipment may help considerably with comfort and efficiency.

Multi-stage equipment may have a better chance of achieving "steady state" (which is good for the equipment) due to the slightly extended run time (target run-time of approx 15 minutes). 

Good luck out there, and protect your reputation. 

Carlos