You can set the form size on load according to the system resoulution.
Suppose you are creating a Form of size 300 X 300 in 1024 X 768 resolution, that means your form width is in the ratio of (300/1024)
and form height is in the ratio of (300/768). i.e.; you have to calculate the width and height by (300/1024) * Resolution.Width , (300/768) * Resolution.Height.
Getting the current resolution by the following code
int sHeight = Screen.PrimaryScreen.Bounds.Height;
int sWidth = Screen.PrimaryScreen.Bounds.Width;
Now you can calculate the form size by
Width = (yourDesiredFormWidth/YourCurrentScrResolutionWidth) * sWidth;
Height = (yourDesiredFormHeight/YourCurrentScrResolutionHeight) * sHeight;
Here YourCurrentScrResolutionWidth and YourCurrentScrResolutionHeight are the constatnts i.e.;1024 and 768 if your current resolution is 1024 X 768.