Setting a button size via script

Hi, i tried to make a main menu for an android game, and found a problem. I wanted to make some buttons’ size(4 in total) to be equal to the canvas height/4, but i don’t know how to set the size.

var sx:float;
var sy:float;

var but:int;
function OnGUI() {
sx=Screen.width;
sy=Screen.height;

GUI.Label(Rect(sx*0.5,sy*0.5,sx*0.25,sy*0.25),"number "+but);
if( GUI.Button(Rect(0,sy*0.00,sx*0.25,sy*0.25),"press me")){but=1;}
if( GUI.Button(Rect(0,sy*0.25,sx*0.25,sy*0.25),"press me")){but=2;}
if( GUI.Button(Rect(0,sy*0.50,sx*0.25,sy*0.25),"press me")){but=3;}
if( GUI.Button(Rect(0,sy*0.75,sx*0.25,sy*0.25),"press me")){but=4;}
}