Disclosed is an emulsion of an internal paper size and a biocidal emulsifying agent, such as a cationic non-starchy quaternary ammonium compounds, that is introduced into a paper-making furnish to produce an antimicrobial paper. The antimicrobial paper is useful as facing for gypsum panels. A method of making the paper includes emulsifying the internal size with a biocide before introducing the size to the paper furnish. A method of making a microbial growth resistant gypsum panel that incorporates the antimicrobial paper is also disclosed.