Protip: Brush sizes, like pant sizes, are not universal.
This is true for brushes made for any of your basic mediums - oil paints, acrylic paints, watercolors. Probably others as well, but I’m specifically talking about fine artists’ brushes. And, obviously, if you have several brushes that are 1 inch brushes, I should certainly hope that those ARE the same - but I’m talking the numeric sizing scale here. Brushes sizes like 000 (also known as 3/0), or 8, or 24.
Most brushes that I’ve seen are part of a line of brushes - that is, several brushes made to be part of a series of brushes. Once in a while I’ll come across one that is just kind of… that one brush, but usually it’ll be part of a line. Many lines of brushes will offer several shape categories - round, filbert, flat, bright, angle, fan, script - to name a few. And within each of those categories, you’ll usually find several sizes.
Generally, watercolor brushes tend to get smaller than others (but still offer a great range of big brushes for your wash needs). In addition, watercolor brushes tend to have shorter handles than those intended for acrylics or oils. This is because most acrylic and oil painters, traditionally, work larger and more gestural (usually painting while standing up, for instance), than watercolorists.
Brushes within a line can vary immensely in size - size 0’s and even some several sizes smaller than that up to sizes that top off around 24. Pick up a handful of 8’s from different lines of brushes and they probably won’t all be the same. Some won’t even be close. But, at the very least, number sizes do all go in the same direction from line to line - the smaller numbers are for smaller brushes, and the bigger numbers are for bigger ones. Many lines of brushes come in even sizes, so if a supply list calls for a size 7 round it may be harder to find than a size 6 or 8. Frequently the biggest brushes will skip even further, e.g. jumping from 12 to 16, or 20 to 24.
These numbers aren’t standardized across brands or lines. That being said, a general size 8 round is a great place to start for a watercolor brush. (I can’t speak for the others, as I’m not particularly well-versed in other painting mediums (someday!)… although, I’ve heard good things about filberts. :B I may delve into brush shapes another day. As well as brush material.*)
Here are some basic rules to get you by when searching for or caring for a brush:
You want a brush that will hold a fine point when it is wet (even damp is a great sign). If your brush has bed-head when it’s bone-dry it shouldn’t matter, as long as it comes to a fine edge or point when wet or loaded with paint.
Frequently brushes come with a little see-through plastic tube covering the bristles. This exists as a precaution to keep the hairs of the brush in good condition while in transport. Once you take the tube off to use the brush, I recommend just tossing the little tube - as tempting as it is to keep it so you can put the little hat back on the brush once you are done painting, there is a high likelihood of creasing hairs and ruining the brush if the tube gets put back on. Hot water can sometimes aid in returning disobedient hairs back into place, but it doesn’t always work.
Do not leave your brush sitting in water. Your brush will not appreciate it. And you will not appreciate having to buy a new brush.
Don’t leave your brush sitting with the bristles up when wet, as the water will be more likely to climb further into the ferrule (the metal part crimped around the brushes). Lay it down flat if it is still damp or, if possible, hang it bristle-down so that gravity can help to expel any water still in the hairs.
*I’ll focus on brush materials another time perhaps, but it is worth noting now that the U.S. is having trouble getting Kolinsky sable hair brushes in. For one reason or another, it isn’t happening, and it isn’t looking good. I highly recommend that if you’ve ever considered upgrading to a natural hair brush over a synthetic, or if there is a particular Kolinsky that you’ve had your eye on for a while, save up and buy it now. No U.S. retailers are getting any new stock in on them, so once they sell out of the stock that they have, there is a possibility that they’ll never have them again.