It's a 14m/m O-ring fitting, so I think it's up around 19 or 20m/m (I used a standard-issue left-handed ISO crescent wrench to remove mine - I know they're not in that tight.)
I do suggest you put some PTFE or never-seez on the threads before you put it back in, and tighten to "three flats past finger tight" (install to finger tight, and turn one-half turn with a wrench.) No more. And be sure to put on new O-rings.