What truly matters is quality, so 1366*768 isn't bad, but if you could get your hands on something that has more than 60hz of refresh rate, that would be really nice. My monitor is 1366*768, tho it's stating to die slowly
The monitor size really doesnt matter. Well for me at least cuz my PC is not very good and I dont really game that much. The only thing that size changes is how much space it is gonna occupy. Of course 18" 1366x768 and 24" 4k would make difference if you look closely but it doesnt matter THAT much in a long run.
I have 19" 1440x900 @75Hz and im pertty happy with it.