Depending if the muslin/calico cotton fabric, is bleached or has been fire proof treated. In which case it will be pre shrunk, because of the water application.
With unbleached raw cotton fabric. the amount of shrinking is not that great, it is only the difference between pull stretching tight, to secure it on to a Flat frame & allowing the calico skin to dry as tight as a drum. After applying a water based, fire proofing, size or paint product & it dries.
I doubt by painting raw cotton drops the shrinkage will be that dramatic, if noticeable at all.
One thing with Leg drops.  is to add enough muslin length, to make allowance for a tail batten pocket and skirt. Along with a fold at the head, to secure a Hessian or burlap webbing reinforcement, that is if the drop head is to be eyeleted & tied of to a head batten!
All so you may have to add an eyelet to the tail off stage edge, to be able to sash tie a sandbag or on to a stage fixing!
I hope this helps?

Good idea Castme about using the burlap webbing to reduce the cloth rolling, I only suggested it for the head of the cloth to reinforce the eyelets for the head batten ties!
I remember in a previous life using this for cut cloths, rather than making up & gluing on a ply profile to stop the cut edges from rolling!
I have been trying to remember the Roman theatre name for a Leg Flat or cut profile Legs, I think it is called a some thing like a ?Coliuse? but can?t recollect how to spell it? [It?s an old age thing!]
Anyhow Aaron you can always just paint the back edges of the drop, to reduce the cloth rolling or curling to a certain extent. But gluing on the burlap webbing, thanks to castme, would work. Also it will serve another purpose of adding weight & stiffness to the drops on stage edge & reduce the Leg from faffing about, even with the weight of the tail batten. This happens with complete Portals [Boarder & Legs as one unit]. When the Leg is moved on entering & exits, & with any movement that is transmitted goes through the Boarder as well!
With the tail batten pocket & fold skirt, or any joining of the muslin on muslin, or whatever, is just use a two part epoxy glue, as it is far easier than sewing!  

Starch!
That?s a great idea, much better than wallpaper size  & possibly just as effective, with out the smell - if you leave it in an open pot over night!
Originally we only used the Size to bond the surface, when using Poster paints, which was cheaper than emulsion paint at the time.
But yes it would work, I have used corn flour when we ran out of wallpaper size, so the starch will also gave a stiffer surface to the cloth fibres.
Bewdy Shatcher

Very good points Shatcher about the fire retardant.
Of course this is one of those things that Pro & Comeatre in this State, don?t worry about.
I have attempted over the years with the Fire authorities to instigate that fire proofing is carried out in theatre here.
But after throwing the question at them, I?m afraid it is like taking to a brick wall & the answers received are just as dense?
It might be the State is too big & they are always prattle on not being able to pay firey?s to  police it. Which is strange as the majority are volunteers anyway.
There again we are that laid back it is too much hassle to get off their bronzes. Anyhow in probably is not a priority as, I believe most are closet pyromaniacs anyway. Which is proved with the majority of bush fire  offenders caught, are actually Fire Volunteers, who are always seem to be first at the fire scene, keeping the fire going until their Brigade mates front up!
But putting all that aside it should be a general housekeeping practice, especially on the rear of sets, costumes & hangings. Including the retardant additive in the paint mix is a great idea, I suppose even putting it in the washing machine with costumes, might be a good solution when using Brand X soap powder?