I purchased the full sized bottle when I ran out of my previous cleaner and am happy I did. It really provides a deep clean of my makeup brushes while still being gentle at the same time.
Another thing I have found useful, but definitely not required, is the brush cleansing palette.
It is a hand held silicone palette/almost dish thing. I find if I put a drop or two of the cleansing gel onto the palette and get my brush wet, then begin swirling the brush on the palette. The gel foams and the different textures help clean the brush. I then rinse the brush and palette and boom, clean brush. I clean my brushes at least weekly and the palette has been great at making the whole process a lot faster.
The palette has a convenient handle on the back so you can keep a grip on it. As mentioned before, it is definitely not a requirement for brush cleansing, but I have found it extremely helpful in getting brushes completely clean.