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Brushes are sized by width. As a rule of thumb, bristles should be about 50% longer than the brushes' width (and even longer on very narrow brushes).
Brushes with natural bristles should not be used with water-based paints; they lose their stiffness. Before being used for the first time, natural bristle brushes should be soaked in linseed oil (not resting on their ends) for a couple of days in order to swell the bristles inside the ferrule, which helps to prevent their pulling out.
Synthetic bristles, recommended for water-based latex and acrylic paints, may also be used for oil-based paints. They can be destroyed by certain lacquers, varnishes, and other paints containing strong solvents.
Ideally, it is better not to use the same brush for both water- and oil-based paint, even if its bristles are synthetic. Cleaning will become a nightmare. If you work with both types of paint, it is best to keep two separate sets of brushes.
There are four main classes of brushes:
Made in widths from 3″ to 6″, in ½ inch steps from ½ to 3 inches, and from 3 inches to 5 inches in steps of a whole inch. The 5″ and 6″ sizes have been almost killed by the rise of the paint roller. It hardly matters for do-it-yourselfers. The wider the brush the greater the demand placed on rarely-used muscles in the hand: the weekend painter is generally more comfortable with a 4-inch brush, and women often prefer 3″. Wall brushes are used for walls, ceilings, siding, and floors. They have a square edge.
A 2″ synthetic bristle, chisel edge, trim brush
These brushes are made in three styles: round (½ to 2 inches in diameter); oval, and flat (1½ to 3 inches). They often have a chisel edge. A 1-inch chisel edge brush would be a good choice for trim; a 2-inch chisel edge could be used for doors, shelving, and window frames.
Short videos 
by a manufacturer of high-quality brushes illustrate the use of round and flat trim brushes.
Flat 2″ to 3″ brushes. Be sure the composition of the bristles is compatible with the varnish. A brush used for clear varnishes should never be used for paint, since it is impossible to clean all the paint pigment out of a brush. By Murphy's Law, some pigment will come out into the next varnish coat.
The softness of the bristles matters in varnishing. The stiffer the bristle, the more likely it is to leave brushmarks.
On the left, a 6″ black China bristle brush. On the right, bristles of Tampico fiber. Both are made by Wooster.
Very wide brushes, 5″ or 6″. The brush on the left is about $60; the one on the right is $13 (2020 prices). The difference arises from the two very different types of coatings that are applied to masonry and stucco.
One is sealant, which is typically extremely thick; the other, paint. The brushes used to apply sealant have bristles of Tampico fiber, or very heavy, stiff, synthetic bristles.
Paint is usually applied with a roller with a long nap. The brush on the left is made for those parts of a job where a brush is needed.
Because the surface is so abrasive, for painting masonry a cheap brush may be a good idea, unlike almost any other class of paint brush.
Good advice on painting household walls: how-to-paint-a-wall-like-a-pro
Another , with less emphasis on prep and more on use and care of tools, some of them proprietary.
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Last revised: 1 August 2020.