I'm running a 16"x18" coil that I purchased off Ebay from a seller just up the road in Wisconsin. Price was great and it's a high quality piece - not sure if the actual manufacturer, but the carton was made in Texas and there is a coil manufacturer down there.
fit between the top of the A-coil and the takeoff for the humidifier, and required a little "tin-bashing" to make the coil fit in the plenum - basically a size-on-size "press fit".
It does restrict the airflow a little bit, but the temp. rise through the furnace when on gas is still well within limits. I plan to pull the HX out and cap the plenum opening in the summer as the fan already has enough difficulty pumping sufficient cold air up to the 2nd floor.
I am feeding average 160 degree water in, and 140 out. Air into the coil is 78 (the humdifier flow warms the air a bit), and 125~130 out. The output air when running gas I measured at 138, so I'm very happy with the coil performance.