Novel sizing mixtures to achieve improved sizing along with other benefits is disclosed and claimed. The invention is a composition comprising a sizing mixture having a stabilizing amount of one or more aldehyde-functionalized polymers and a sizing amount of a sizing composition. The invention is also a method of improving paper and paperboard production and enhancing sizing through adding an effective amount of the disclosed sizing mixture to the paper machine and a method of producing a medium having cellulosic fibers, wherein the method includes adding the disclosed sizing mixture to the medium at any point in a papermaking process.