The photos I've seen have a 2:1 mainsheet.i would guess that the boom when fully out would give a run of no more than five metres, so 10m would be needed, plus a tail of perhaps 3m. 15m should do it, and you could cut any extra off and use it for disciplining the crew. Please don't spend any money until you have checked my guesstimate.
pretty sure mine has a double tackle which would be 4 to 1 - amazing how something so familiar can be shrouded in mystery when you try to remember! yup - just looked on an old photo of SWMBO in the cockpit and its definitely 4 to 1
I wouldn't have thought anyone will have the figures to hand.
Work out the distance from the traveller to the boom end at maximum boom out, multiply it by 4 or whatever your purchase system is, usually its 4 on a boat this size. And then add a metre or so to give you something left at the end.
I agree that 4:1 seems most likely but if you google Trapper 500 and view the images, there are several which look like 2:1, which puzzled me. I can't paste one in from my pad.
My Trapper 500 has a 4:1 block. In my opinion a 2:1 block would be very hard work. Distance between blocks with boom centred is 1.3m. However, you would need to know the distance with the boom fully out - sail reaching the shrouds - to calculate the length of rope needed.