I demonstrated and described the role of sizing in paper during our last office hours on Friday. To watch the replay, here's the direct link:
The link below provides some general information and a couple of recipes. Their gelatine recipe includes some specialized ingredients, so consider the wheat starch-based one instead, and you can try starches other than wheat based on what you have in the house, eg corn or rice starch, just bear in mind that your results will probably vary.
I will update with at least a couple more resources/links as time allows. If you come across something great, please let me know and I will share it here!