help with passive idea

[MACnonsence6]

saw that fixmyspeaker makes passive 18s and there not to badly priced. thinking about about getting one with an re sx 18 just wondering how i determin what size box i need when using a passive is it the same as a ported box or? also how do uk how much wieght tunes the sub to what fq

in doing some reading about passives ive read that when doing a passive set up you want twice the cone area that way you dont over exert your passive like for one 12 you would want 2 12" passives atleast this is what ive read so far now looking at the re sx it has an xmax of 22 now idk if all of that is usable or not but the 18" passive from fixmyspeaker has an xmax of 65mm. so im asuming becouse of the passives much higher xmax i should be abble to pair the 2 with no problem still reading up on things though. really wana do a passive set up and no not becouse of meade becouse i have a set of old school towers that use passives and i love the idea