It's 3/16"od inside of 3/8"od tubing.

n00b asked... does using a vertical coil not trap liquid?
It could if the velocity was very low, or the annular space was large, but in this situation neither is true (it was also much easier to have a vertical orientation due to the size and shape of the space available).

However, you are correct. Ideally it is best to have a horizontal stacking of the heat exchangers, just to insure that there will be no liquid hang-up problems when returning to the compressor.