Sizing paper ready for making a woodblock print.

Sizing Japanese Washi paper. 

I've done some reading up on the subject, notably via www.woodblock.com and this page all about sizing recipes.

Essentially sizing has nothing to do with the dimensions of the paper, but actually means the coating that's applied to it in order to facilitate better colour retention and prevent ink spreading and feathering out as soon as it touches the surface. It has the added advantage of (when being put on the back of the piece of paper too) creating a bit of a barrier against the abrasive action of the barren when it's being used.

This page - http://www.paperslurry.com/2016/03/10/sizing-and-paper-watch-this-to-learn/ - has a good gif on it showing the difference between sized and unsized paper.

And here's a tutorial on exactly how to do it. Seems pretty simple really - https://whereispaperwoman.wordpress.com/2015/03/13/paper-sizing-101/

Here's a photo culled from the Internet showing some paper being sized

Seems to me that getting some wheat starch from the local supermarket may be the best option for making some Sizing agent at home. I've no idea where you'd get Alum from.