I said it before, Arrow has done a good job at copying previously existing features and stories from other major comic books and movies.
Yes, the plan was originally to have Ray be Ted Kord, so shrinking was not an idea being worked in, but we are at the point in the season where his character and abilities should not be defined by who he was supposed to be. So I really think that they're postponing his shrinkage until either the Finale, or the Spin-off, which will be filming and getting post-production well after Ant Man is out.