Sketchup uses inches, but provides methods so you can use any measure you like -> http://code.google.com/apis/sketchup/docs/ourdoc/numeric.html
val = 1.m # 1 meter
pt1 = [0,0,0]
pt2 = [val,0,0]
pt3 = [val,val,0]
pt4 = [0,val,0]
my_face = entities.add_face [pt1, pt2, pt3, pt4]