This should get you what you want -- although its set for the primary screen.
begin
for line$ from fin(0,"XYMonitors")
read data from line$,sep="," to *,*,*,wd,hi ! Get Primary monitor size from 1st line
break
next
print 'dialogue'(0,0,80,25,"",opt="F*m"),'SR','_black','white','DF', ! Create the window
print 'option'("AlwaysOnTop","1"), ! Force on top
print 'option'("Frame","none"), ! Remove frame
print 'option'("XYPos","0,0,"+str(wd)+","+str(hi)), ! Set size to full screen size
print 'CS', ! Clear the screen
escape
You code combine all the prints to a single line but it was easier to read by breaking them down.