Scaling/size issues in delta printers
Issues in X-Y size on a Delta are usually the result of an incorrect diagonal rod value in the firmware.  This should be easy to fix assuming the rods were built together.  

The formula for this is (20 in this case is your test print X-Y):

New DELTA_DIAGONAL_ROD = 20 / measured_length * Original DELTA_DIAGONAL_ROD

I found this information (here), this site also has more information on setting the radius and other delta specific issues.

If you encounter issues with the size of the rods you should build new ones using a jig.  There are several (here's one) I found on Thingiverse but they require printed parts and they do all the rods horizontally which I think would be tough to keep equal. I would use something similar to this picture but with a longer rod/bolt so you can fit all rods on the same time.  When I did my Kossel Mini I used a piece of the extrusion with bolts coming off of it to keep the rods consistent.



For the other issues you mention you should open another question specific to each issue.