Questions questions! Please understand that in these final stages of development we're juggling lots of different issues and variables related to sales and positioning, so it's hard to be absolutely precise and anything I say here may change suddenly. That said - Answers answers:
At the moment we don't have a retail store version in development, but we have looked at ways we might change that in the future. As far as we know currently (these things can change at any time) ArtRage 3 Studio & Studio Pro will only be available at launch directly from us.
Live brush sizing? Yup.
Educational licensing: We're planning on continuing to do educational licensing for the new version, the exact prices and models have yet to be finalised.
Regarding AR2 licenses after upgrade: If you upgrade to ArtRage 3 in either form from ArtRage 2 your original copy of 2 won't stop working and can be run on the same machine. Our intention here is to make it clear that your key is non transferrable, so you can't upgrade to 3 then pass on the 2 key to another user, as that key has already been registered to you and cannot be used to upgrade again.