To create a figure window that is one quarter the size of your screen and is positioned in the upper left corner, use the root object's ScreenSize property to determine the size. ScreenSize is a four-element vector: [left, bottom, width, height]:
scrsz = get(0,'ScreenSize');
figure('Position',[1 scrsz(4)/2 scrsz(3)/2 scrsz(4)/2])