The wider and higher the better, there are some 'standard ' sizes of pre cut cover 'Visiqueen' is one ,( £100 or so +vat ) 7.2m x 14m I reconstructed two pollys some years ago , 4m wide and 2.7 high, and just short of 14m ,1m spacing with top centre joining rod. and timber / plastic , heavy end sections. they were donated metal hoops . Plastic will require some strengthening. Its a good idea to put the sheet on in warm weather as it expands quite a bit .