Chris Green formulated the question :
Is there a size of pipe above which a syphon won't work because the
water empties out of the down pipe without 'sucking' the water above
down? It works well with small pipe because, I assume, surface
tension helps to prevent the water from 'dropping out'.

Correct!

Air has to be prevented from being sucked up the discharge pipe, by
having the discharge under water, or the syphon will be immediately
lost with a larger pipe.