I didn't use any sizing in my tests. Small prints went fine, but I ran into streaking with large (12x20) prints. I was using Lenox. To deal with the streaking I would try other papers rather than use sizing. Since the process has a lot of similarities, it also might be worthwhile to try an oxalic acid pretreatment as can help papers used for Pt/Pd. I think Sandy King lists several papers that worked well for him in his Kallitype procedure--probably written up on unblinkingeye.