Many times I am asked if I will "size" the walls. When asked this, I have an urge to explain what "size" is and how it differs from modern day wallpaper prep-coats. But, initiating a dissertation on this subject may not be welcomed or appropriate. The customer needs to know the job will be done right without a discussion on the definition of terms. However, for those who are curious about "size" and "wallpaper prep-coats", please read on.
"Wall size" is a term that once had a specific meaning. Through the years, it has been so generically used that it has greatly deviated from its original definition, which now contributes to confusion. Like the trade names "Xerox" and "Kleenex" which have come to mean any photocopying process and any facial tissue, "wall size" is now used to refer to any and all prep-coats applied prior to installing wallpaper.
But let's hearken back to your grandmother's wallpaper, which was real pulp paper installed with wheat paste on plaster walls. Wall size was a powder made from wheat or animal products; it was mixed with water, and applied to the walls in preparation for the wallpaper. It was (and still is) a diluted paste. It was used to prevent absorbent plaster from drawing off too much paste from the paper and to give smooth plaster some "tooth" to better hold the paper. It was not formulated for use over modern materials such as gypsum wallboard, joint compound, builders' flat, vinyl latex, and many other porous and/or water-soluble coatings. It would not enhance adhesion to glossy sealed surfaces. Also, being starch-based with no biocides, it is inappropriate for use under coated wallcoverings as it could support the growth of microbes. In short, wall size is inappropriate for use in the most construction today. Modern wallpaper prep-coats have become the product of choice with contemporary materials.
These coatings are formulated for specific purposes. Unfortunately, one size (or more correctly, 'one prep-coat') does not fit all. There are prep-coats that can seal and prepare highly porous materials such as builders flat and gypsum wallboard, there are prep-coats that will allow wallpaper to adhere to ceramic tile and other glossy surfaces, and there are prep-coats that will do very nicely over common everyday painted walls. A handful will satisfy a number of requirements. Some are solvent based and some are waterborne. But not one is "wall size".
After considering the wall surface and the wallcovering, one can choose the best prep-coat which will enhance how the installation will look, how long it will last, and how easily it can be removed when time comes. And yes, once in awhile "wall size" will be the best choice.