Image Size & Resolution Calculation

480 / 640 = 75%

600 / 800 = 75%

768/1024 = 75 %

y = x * .75

If I have a 15 inch Monitor

x

y

z

z2=x2+y2 or 152=x2+.75x2

225=1.75x2 or 128.57=x2

x=11.3 & y = 8.5

So what effect does the size of the monitor have on resolution ?