For my clients, in every city I am set-up in I have an agent who pulls all the comps and writes up an opinion on the value based on size, area, finishes, market. 

They take their portion of the housing voucher like clockwork and my funds are tied up in their system.

or in the 700's because most people think they have perfect credit.You also have to know occupancy type like primary, vacation, investment, how many units, LTV/CLTV, Term, Fixed or Variable, Prepay or No prepay, Loan size etc.

I would use all 3 plus look at properties currently available for rent that are similar to what you are trying to comp (3 bedrooms/2 baths; similar condition; size; etc). 

There is no one-size-fits-all for markets and salespeople.

It is a 6 bedroom and while I see a couple of sales in that 1.6 to 1.8 for 5 or 6 bedroom in that size, that seems to be few and far between.  

A nice but rental quality unit gets 1500-1650 regularly in that area depending on the size and finishes.

It totally depends upon the size of the STR. 

This question is definitely very location dependent - licensing across state-lines is a tricky thing to navigate so depending on the size of the lender they may operate in one metropolis or all over the country.

The point being: it can be just about any size you want.