You will see that the size of a brush is indicated by a number printed on the handle. Brushes start from 000, then 00, 0, 1, 2, and up. The higher the number, the bigger or wider the brush.
Unfortunately, there is little consistency between brush manufacturers as to what these sizes actually are, so a number 10 in one brand can be a different size to a number 10 in another brand.
I generally use something like a No. 16 round for watercolour washes, with perhaps a No. 6 round for more detailed work.
before ordering from SAA, have a look at various brushes in a local art materials supplier, and note the sizes you feel happy with, then order those as a start.
If you look through the SAA catalogue, you'll find some excellent deals which offer 3 brushes, with the cheapest of them free.