Hi! I'm about to buy a new computer and at the same time, arma 3 and cs go, I'm gonna concentrate on arma 3 for now. But i started wondering if a 24'' monitor is too small for example spotting enemies, because of all the long range shooting. If anyone could please respond I would be really greatful, thanks :)

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.

Or, do like me and just accept you need glasses (but I havent gotten any yet). I have just accepted I dont see shit in any game.

I think selection of monitor size is similar to television. There is a recommended maximum size for television for sitting distance. When you sit to close to a large television your eyes can't see the full image and it's tiring to watch a long time.
When you sit at your desk you are abaut 80 cm away from the monitor. I play with a 24" monitor and I have to focus my eyes (and turn the head a bit) to the cornes to see what happens there.

Okay, thanks everybody for your help! :) Really helped me deside. :)