The plus side, lumber is cured rock hard (sometimes not so good to work with) true 2x4 or dimensions.

I wanted to show more dimension than a picture does.

There are NYC guidance documents that state dimensions.  

Habitable rooms, not including the kitchen, are required to have a minimum horizontal dimension of 7 feet.

@Trevor MillerWelcome to BP.Your first stop (either virtually or physically) would be at the municipality to confirm whether the property is zoned such that building a duplex on the lot is possible and whether the lot dimensions are sufficient for two units.If you make it past that step, then you need to decide what kind of building you wish to construct and what you can afford to build:  i.e. minimum code compliant building; R2000 rated building; Passivhaus, etc.  

The dimensions don't work in my opinion.

The selling price is $35,000 higher than my unit and it was also flooded, same dimension, etc. 

Any idiot can do a google search of solar panels on a home and see that the dimension you quote are ... absurd.

Should I order custom windows to give myself exact frame dimensions of the old aluminum ones or should I save 20% by buying stock sizes that are fairly close to the old ones?

Few years ago I was asked by several out of town investors to manage their investments and thus Dimensions Property Management was founded.