you can programmatically set the size of the form by modifying the width/height property or the Size property:
this.Size = new Size(width, height);
but you may have to also make sure that the controls on the form scale/adjust themselves accordingly, to do this I am unsure but know there are a few amount of topics about this on these forums, so do a search and see what comes up :-)
You could also just not allow the user to resize the Form if you like? To do this, set the formborderstyle to FixedSingle
Does this help?