I have used the same shape of tarp for years.  The 12' is tight for 4 adults in a driving rain storm but it works.  If you get a really bad storm put your canoe on its side on one side to block the wind and rain, it ialso make a great place to store your gear.  If it does not have one glue a small loop in the middle so you can hang a candle lantern.  If you get Eureka's 8 foot 4 piece poles (very light) you can use 3 sections to keep the tarp low in a storm the 8' works great otherwise.