Brush selection is basically broken down in several factors:
Paint type to be applied
Size of project
There are two different bristle types: Synthetic and Natural. The synthetic variety are far more durable than a standard natural bristle brush, and due to the way they are manufactured they do not absorb vast quantities of water, which leads to swelling in natural brushes.
Natural bristle brushes are far more absorbent, and they can produce satisfactory results with any solvent based paints, however, they may not be so well suited to water-based paints.
In regards to the size of project, always follow the same general working ethic :- use a larger brush (between 2”-3”) for the main areas of the room, whilst using a smaller brush (1”-2”) for the more precise areas such as skirting boards and door surrounds.