Im running 21mm front 18mm rear.  Its a nice combo for me.  Stiffer then stock, so it makes for better handling, but also more of a neutral handling then factory which it  typically set up with too much understeer.   This changed the bias to add a bit of oversteer by stiffening up the rear more then the front.  Its a very streetable upgrade to me.  Its def made a huge difference.  Bigger might be WAY better when it comes to auto-x and what not, but you need to be careful about going too big for all year around daily driving.  What might be good for auto-x on dry tar might make for poor handling in the snow.  I might eventually go a tad stiffer but like the change for now.  If i do go stiffer in the future, I will try to pick a setup to keep a more neutral to slightly more oversteer as I have now.