Resolution is more important than how big the monitor is, but I assume you just got a 1080p as usual. Even if it's lower than 1080p you shouldn't have any problems with spotting, it doesn't really depend on screen size, if it was all the pros whould be playing with a projection device

. I have a friend with a 900p 19" monitor, and while I'm on my 1080p 27" setup, he's still a better spotter than me. On long range shooting screen size doesn't really matter, you'll realize as you play that it's the visual equipment like binoculars or just general experience of keeping an eye out. ArmA is all about experience and learning, hardware can only help you with flying or getting good fps.