Hi John,
Yes size does matter. At least when speaking about MD'ing and depth and sensitivity. Hmmmm ok...well trying to make this G rated.
I have a TDI as well. The 12" circular coil is junky.

The smaller the coil the more sensitive to smaller gold but less sensitive at depth. Always keep in mind there is a trade off, and you need to know what size targets you will most likely encounter in any specific area. Yes we are all thinking of finding 1 oz nuggets but its not realistic, unless you are in an area that is known to produce. If you are in open land like in Northern Nevada, a larger coil is much better to cover more territory. But if in rocky hilly terrain, large rocks....well it gets harder to work the coil in the nooks and crannies around rocks. A elliptical coil 6x10 works well. I have a 17 x 11....which is huge and also a Miner John 5x9 which also has much better noise characteristics.