Well, try not to short change yourself. I think the Cuben fiber weighs something like 0.5oz a square yard, so the changes you make in size may be of greater value to you than the weight. Personally, I wouldn't care for the 8' length. I like something longer since a 6' person only gets about a foot of coverage on each end (I prefer a 10' length, but that's just me). Another thing, if you go a little wider, you might even have enough room to share your tarp if the need ever rises. So, lets say you went with the 6'x8' instead of the 5'x8', that's like 0.44oz of extra weight or, ahem, protection.

So what do you think??
Eric