I built a large delta almost 12 months ago based largely on this "Delta 6" here http://openbuilds.org/builds/delta-six.476/
It has worked very well, and it was not a difficult build, although the original person has left so the info is spread over many pages of discussion along with file updates. I used the full size openbuild wheels and openbuild plates on mine as carriages (others have done similar), I have a 300mm build plate and can print 480mm tall, I used 1m verticals
One of the more recent builders of that delta-6 re-did all the files and made improvements on many of them, and this is a link the the build that has his files http://openbuilds.org/builds/delta-six-jerry426-variant.4005/
I've built a smaller kossel out of 2020, and found it ok, but I can get flex out of the 2020 that you don't get out of the larger 2040 extrusion used in the delta-6, not really an issue on the kossel size but for me I wouldn't like to use 2020 for anything larger than a std kossel size- in fact for my next build I have started I'm going 1.5m tall verticals and will use 2060 extrusion