A round bale forming machine comprising a frame supporting substantially circular rollers at opposite sides of the frame supporting inner and outer portions of an endless flexible apron arranged in a substantially C-shaped configuration upon the sets of rollers with the ends of said configuration spaced apart to comprise an inlet for crop material to be delivered from a pickup head to a bale-forming compartment defined by the inner portion of the apron, drive sprockets on the frame positioned in one end of the C-shaped configuration of the apron adjacent the top of the inlet and takeup guide sprockets on the frame positioned in the other end of the C-shaped configuration of the apron adjacent the bottom of the inlet, the takeup guide sprockets normally holding the portions of the apron taut but being yieldable to permit the inner portion of apron to expand a limited amount against the action of the takeup guide sprockets when a bale of desired predetermined size has been formed, and signal means on said machine interconnected to one of the takeup guide sprockets and operable when the guide sprocket is moved by expansion of the inner portion of the apron to render the signal means operative to alert an operator of the machine that a bale has been formed to desired size.