As Mike points out, if you are taking the original images of the buildings to be used on the layout yourself, it is wise to employ a "story pole" which is basically a rod, or narrow piece of board usually 6' tall, that has been heavily marked off at one foot intervals. This is placed against the wall of the building such that it is included in the photo as a precise size reference (easily removed later by Photoshopping).
When using existing photos where a story pole was not present it is the usual practice to assume that a common entry door opening measures 6' 8" as a general scaling reference. This will, of course, pertain basically to modern buildings and codes, as this dimension did vary widely in the distant past (as did that of window openings floor to floor).
A particular caution necessary in utilizing photos of multistory buildings is "keystoning" of the image, an effect where portions of the building higher than the photographer will appear increasingly distorted. Historic photos often were taken with this problem in mind and the large view cameras of the day were positioned such that this was avoided. Photoshop has an application that, with care, can largely rectify keystoning.
NYW&B
