all ratios are relative to screen size....
so for a 120" screen, you want to sit 120" away (1:1 ratio).
a 1.5:1 throw ratio for a 120" screen means you want the projector 180" away (15ft), 2:1 = 20 ft...etc.
the limiting factor for a big screen is obviously light output, and you definitely dont want the projector in front of you (so at least more than 1:1 throw ratio is what you want)..
some people feel like 1:1 is too close to sit, so you need to see what feels best for you....keep in mind that sitting farther away decreases your eyes effective resolution of the screen (ie, just pulling numbers out of my arse, but from 20ft away 1080p and 480i look the same on a 60" screen -- totally random numbers for an example, definitely not accurate)....follow?
there are numerous calculators for all of this online, they may seem daunting at times, but just work with it and fiddle and it will start to make sense.