Water size is the glue used to adhear the gold leaf in water gilding. It is made from gelatin and there have been several different recipes come and go through history.
I use pure gelatin in the form of empty capsules to make water size. The usual mixture recommended is one capsule (size 00, 95ml capacity) to 250ml (half US pint). I often use a slightly stronger mix of one to 150 - 200ml. A stronger mixture is more difficult to clean up but I feel more confident of its adhesion having restored older glass gilding where the gold easily peels off.
METHOD: Separate the two sections of the capsule, place in a pan with a spoonful of cold water and briefly soak to soften. Add some hot (not boiling) water and stir until completely dissolved. Ensure that none of the gelatin has stuck to the side of the pan. Add cold water to the desired strength. There is no need to use it hot or warm.
ALTERNATIVE: Add capsules with correct quantity of water and heat over a flame. Stir constantly until it heats enough to dissolve the gelatin, then remove from heat. Do not boil, it weakens the size.
Very strong size will cause a cloudiness to develop in the gild which worsens with age. A weak size will lift and bubble with the second gild when patching holes. When the lettering is cleaned up, it is also likely to suffer chipping if the size is either too weak or too strong.
Gelatin capsules can be bought from pharmacies. It can also be bought from gilding supliers in sheet form or as diamonds. You can also use crystals (cooking gelatin) but I am not confident of its purity. Some older recipes call for the gelatin to be boiled for several minutes. It seems this was probably a raw gelatin that needed boiling. Either that or they didn't know that gelatin glues break down with re-heating at boiling temperatures.
The water quality is also important for a clear bright gild. Poor quality water results is some cloudiness. Some practitioners use only distilled water. I find that Melbourne's water supply is satisfactory as it is a soft water (low mineral content). A high sulpher or chlorine content could spoil silver or low carat gold.
There have been other ingredients for water sizes used through history including:
Cennino Cennini describes a couple of different water size recipes. His prefered one is an egg white size made by mixing egg white and water, standing overnight, discarding the foam and then straining the resultant liquid. This glair was also commonly used as a binder for pigments used in medieval illumination. If you think this must be like egg tempera, you're partly right. Tempera was usually made with the yolk. Because of its much higher fat content, the yolk dries to become water proof and is more robust than glair.
Reportedly a 16th French curator/restorer bemoaned the use of glair size as it appeared it was failing in a large number of cases. His preference being for a vegetable source of gelatin.
It should be noted that egg white is more commonly known as a resistive to gilding. Sometimes you want to prevent gold leaf from adhearing to surrounding areas during surface gilding. The egg white is painted on these areas, allowed to dry and then cleaned up with a damp cloth when the job is done. I find talcom powder to be an adequate resist in most applications.