An old 300cc, manual, 2 wheel drive happily pulls a 5ft trailer for poo picking at my local RDA group. It cannot cope with boggy ground though. I also use a 400cc, automatic, 4 wheel drive which is more idiot proof but has a crap turning circle and cuts up the ground when doing tight turns. It can however plough through wetter ground than the 300. It still got stuck last winter even with only a half load.
Both are petrol and the 300 is far more economical. For a small trailer on decent ground you do not need to get a huge engine size. The only documentation that came with the new one was a handbook/manual, neither are road legal. The automatic is a lot slower reacting to the throttle than the manual and needs a heavier thumb.