Unfortunately, it doesn’t seem that there is very much standardisation when it comes to brush sizes between different manufacturers. If you use a size 4 of one brand, chances are a similar sized brush from another brand could be labelled as a 2 or 8 or whatever. If you’re purchasing from a shop where you can handle the brushes that’s not really a problem. If you want to order online however it means that some guesswork is involved, and that can lead to expensive mistakes.
The brushes I use the most are hog brushes from Zecchi. I also use their sable brushes, although mostly for sharpening up finer details towards the end of a painting. I have been very pleased with the quality of these brushes and have been using my current set for 3 or 4 years now. I have replaced a few sables along the way, as they are far more delicate than hog hair, but the hogs seem to keep going and in fact get better with age as they soften.
It’s not ideal, but here is my attempt to illustrate the sizing of some of the brushes I use the most. Hopefully you can guess where different sizes are likely to fit in to this scale should you wish to order some. The brushes in the photos have had a lot of use, and still look and work well. In my opinion, the pricing and delivery costs are pretty competitive with other brands of similar quality too.