**DISCLAIMER** This is just my opinion from my past experience, others may agree or disagree:
From what I have experienced, a smaller DD is better than a larger DD for me. Bigger coils can go deeper as we know, and cover more ground per sweep. However, I have found that a bigger coil will have a harder time picking up a smaller signal. To me, either would be good for relics/coins. If you are looking for gold nuggets, which can sometimes be very small like a BB, a smaller coil is better for that. the 8.5x11 DD is a fantastic coil, as is the Excelerator. We also know that in trashy areas that could be littered with pull tabs and can-slaw, a smaller 4x6DD is the ticket because it can pick out a dime sitting 2" from a pull tab. On the other hand, if you are deep in the woods in an area that could have been a civil war camp site, a medium size coil would be fine (this is assuming that in the deep deep woods, you would hardly come across a twist cap or soda can.
Having said that, if you are cache hunting, a larger coil would be great. Because you will be looking for a jar of coins that someone buried 2 feet deep!! What I am attempting to say, is that it really depends on the location, terrain, and what you are looking for. This is why many people have (and bring with them on every hunt) an assortment of coils to choose from.
Sorry it wasn't the answer you were looking for, but this is just my opinion. Good luck!
And don't forget this tip: When you find a target and dig it, after you pull that nice coin out, rescan the hole. I have pulled several coins from the same hole this way. They could have spilled their whole pocket at one time!