Ok - having never lifted a piece of these boards before, I'm wondering how heavy they are - I understand they are heavier than they look, but I'm wondering if I can get by with placing three sheets per wrap of DPM (I'll then bin line on each end and tape up). These sheets are approx 5' x 2.5' - the length of the roof is 5 metres (and of course 5 metres on the other side).
With my excellent non-technical estimating abilities I would guess that a 3' by 10' sheet is about as heavy as a bag of cement (25kg) the last time I had any to dispose of. I snapped each sheet into three so they were managable and wrapped them in DPM by lapping each of the three sections and then folding the ends over on opposite sides. That used the least amount of DPM whilst keeping each 'package' managable by one person. With that in mind I think you would be better off placing two sheets per package as three might get a bit heavy to handle without damaging the DPM.
I disposed of an entire 6x3m roof with one roll of 4x15m DPM by breaking into 1m sections. If you broke yours into 2.5ft2 sections then you would only be able to wrap three across the sheet but your area is slightly smaller than my roof was so you should still be able to arrange them correctly. If in doubt buy two and return one if it is unused.
To snap them cleanly you will need to cut a strip of DPM twice the width of the sheet and fold it over it, give both sides of the sheet a quick spray with a hose and then place a large chunk of wood on both sides of the snap point while leaning one end of the sheet against a wall. Then just lean on the wall-facing part and give the other side a firm nudge half way up. It will snap cleanly along the wood most times.
(BTW, use a suitable vinyl tape to seal the packages up as cloth tapes (duck/gaffa tape) won't bond to the DPM for more than a few hours.)