"Did I read correctly that "6. size the drop" is essentially painting the muslin with latex paint after you stapled a backing on it ?
Wouldn't that defeat the purpose of using it as a backdrop, and having it easily portable?"
That article was referring to painting a piece of theatrical scenery which is normally described as a "flat". If the flat is to be painted to represent a scene, it must first be sized so it will evenly accept paint.
Muslin is similar to canvas in most ways;
Canvas Vs Muslin: Studio and Lighting Technique Forum: Digital Photography Review Canvas is typically of a heavier weight (and thread count) than muslin.
Otherwise, if an artist were going to paint on canvas, they would first size the canvas. This serves two functions; 1) it shrinks the material which removes any wrinkles and sags and, 2) it essentially primes the material making it easier to apply paint or dye to a specific area. Without the sizing the raw pigment or dye material would wick into the fibres of the fabric and it would be impossible to create a sharp edge to a line.
What you use for the sizing is up to you. Theatrical scene painters most often use a watered down white glue mixture. Some may use some left over white latex paint if they have it around the scene shop.
The glue is plentiful in a scene shop, dries quickly and is lightweight in application. You could simply use water but that wouldn't prep the material for painting unless you wanted to simply use washes of color for, say, a backdrop. A "artistic" painter would typically prep their canvas with water based gesso, mostly again as a convenience since it's what's at hand and they are working on a much smaller area than would be a scene painter.
Muslin tends to be lighter weight than canvas which also means it is a bit more translucent than is canvas. Scrim is basically a very open weave, very light weight muslin. You choose your sizing based on how opaque you want the final product to be. Unless we were going to create a painted backdrop I never saw a scene painter size a scrim. But it was normally hemmed at the bottom and a chain would be inserted into the sleeve which made it easier to hang and stay in place.
Painting a scrim or a muslin backdrop normally would require you tack it down to a floor and then size the material before applying any color. Once the sizing has been applied and allowed to dry, the material will not continue to shrink. If you wanted a finished product that is reasonably flexible, then you'd probably do a very thinned down sizing coat and apply color with dyes rather than pigmented paints.
Therefore, weight and portability depend upon selecting the correct materials for the purpose.
"Shrinkage is a concern... any way to tell if it's been sized or not?"
If you're buying from a theatrical or photographic materials supplier, they should inform you of the product you are buying. Most higher quality fabric shops can tell you whether the material has been sized as it comes from the factory. Though, if you just walked into, say, JoAnn's Fabrics and bought a piece of muslin off a roll, I'd assume it
to not be pre-sized.
Otherwise, you can sort of guess by the feel of the material. Or you could take a marker and try to draw a clean line on one small corner of the material. Sizing should make it easy to create sharp edges. Un-sized material will show the inks wicking up into the fibres.
You could then take the chance on wetting the material if you so choose. Or simply take a brush or a vacuum cleaner to any loose dirt on the material and never allow it to get wet. That's your safest bet.
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