Size is a liquid applied to a painting surface such as canvas, wood, or paper used to fill the pores of the fibers and seal the surface to make it less absorbent. Starting a painting begins with the steps of selecting your materials and support, and preparing them to receive paint. Sizing is the very first step in the preparation of the painting support. It is not a coating or independent layer but rather a layer that penetrates into the pores of the support fibers, sealing them to keep the paint from coming into direct contact with them, making them less absorbent.
Particularly if painting with oil, the painting surface must be sized before applying the priming or ground coat to protect it from the acidity and rotting effect of the linseed oil as it oxidizes. Sizing also prevents the oil from sinking into the canvas and causing flaking and cracking.
Note: Paper is typically sized by the manufacturer to help keep the color on the surface on the paper, not to protect the paper from the paint. Paper still needs to be sized if you're going to paint on it with oil paint.
Even if painting with acrylic, sizing helps. Although acrylic grounds and paints won't rot the canvas and could be applied directly to the canvas, acrylic paints stay wet for a long time and can leach organic material out of the canvas and cause the ground and paint to become discolored, called support induced discoloration (SID). Sizing helps prevent SID as well as prevents the support from absorbing too much of the paint into the fibers, causing the color to lose its intensity.
The traditional type of size used since the Renaissance—which was the only type available then—is glue size made from animal hides, such as rabbit skin glue (RSG). RSG has good adhesive strength and also serves to shrink and tighten the canvas, providing a good taut surface on which to paint. It can then be sanded to a smooth surface for fine detail in painting.
Rabbit skin glue comes in crystals which you prepare by soaking in water and then heating. It should only be used under oil paint as acrylic paint will tend to flake off a canvas prepared with rabbit skin glue.
Enough rabbit skin glue should be applied to seep into the pores of the canvas but not enough to create a layer of paint film. The sized surface can be lightly sanded when dry to make the ground layer adhere better.
Rabbit skin glue has some drawbacks, though. It is hygroscopic, meaning that it absorbs moisture from its environment, causing the glue to swell and shrink continually as the humidity changes, which over time can cause an oil painting to crack.
RSG obviously also uses animal products, which many of us like to avoid.
There are several good modern substitutes for rabbit skin glue that are better choices for both oil and acrylic painting: