What kind of evergreen shrub measuring no more than 3 feet in diameter and no more than 4-5 feet high works in a sunny dry location to screen outdoor air conditioning units that is also deer resistant and are not messy (i.e. low maintenance)? I live in Towson, MD.
Junipers should do well in that location. There are many, many varieties. Read the labels carefully to learn the ultimate height and width. Often the size given is "with yearly pruning". So, unless you want to prune, realize that you don't want that!
You probably want a dwarf variety, because they stay small--but they also grow more slowly. There is no way to get around that.
What do you think about Dwarf Japanese Cedars "Cryptomeria Japonica Globosa Nana" ?
These are not as tough as junipers but are wonderful plants. Though they are supposed to be drought tolerant, that usually means once well established. Water the first 2 years at least during dry periods. Plants want about 1" of rainfall a week. They like soil to be about as moist as a wrung-out sponge. The top 1/2" of soil can dry out between waterings, but check soil periodically and supplement rainfall. Be especially careful to watch out for spring and fall droughts.
Don't use a sprinkler system or water overhead. Water around the base of the plant. Here are good tips for planting: http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG24_Plant...