19 is min area. For the last product, there are 12 waste atoms, 4 atoms for the product, and an arm base, which requires 17 hexes. With a single arm you need at least 2 empty hexes to move the product off of the bonder after the last outer bond is made. With 2 arms it may be possible to only have 1 empty hex, this seems harder but it is potentially much faster.

I'm pretty sure using both inputs is the best option. Using the mors input only requires a Berlo, which is much worse for area. Using the fire input only might work, but only allows for 3 track hexes to make space for anime and a calcifier. Managing the anime with only 2 free hexes also seems much harder.