Well, let's take a look at the Jamestown site, which seems to have good bang for the buck right now.
I am looking at the 150" 16:9 and 2.35 screens
Both are available in AT versions using the Stewart 4k material. Good stuff.
16:9 - 150" viewable 74" by 132", total with framing of 81" by 139"
2.35:1 - 150" viewable 59" by 138" and total with framing of 66" by 145"
That says both screens are about 11-12 feet wide.
The 16:9 is about 6' high (viewable area) and the 2.35 is about 5' (viewable area)
You probably can't mount this much less than a foot from the ceiling, so you are talking a foot off the floor for 16:9 and 2' for 2.35. If you work hard, you might be able to raise it another 4-6 inches.
Since you want 2 rows, it will be very difficult to get you a riser that will give you unobstructed view down to 12" off the floor. This is what I meant when I referenced height (vs width) limited. You are definitely height limited here. I would either drop back and consider 1 row, or go with a smaller screen. 18' is definitely marginal for 2 rows anyway. Figure the typical depth of a second row riser for recliners is 7', that leaves 11' to the BACK of the front row. Given a foot of padding and head, that's 10' to eyeballs.
My setup is 11' to eyeballs for a 120" 16:9 screen, and although I like it, some think even that is too big. If you want something that big, look at a single row, spaced 13 feet back and then perhaps a bar area behind it.
I am not advising on projectors here because A. There is a whole forum on projectors under $3k you should be looking at and B. - my setup is now 4 years old, so I really don't have a valid opinion on current equipment.