I used the 5" DD on my Tek. Omega at the Oregon beaches in the very trashy areas around the fire pits, etc., and I used it exclusively on the Omega all September long in 2010 and plucked 895 coins with the 5" DD alone.
On the IDX Pro you could have used the 4½" Snooper coil [size=small](labeled the Blue Max 350)[/size] but measures about 4½" across the lower portion of the coil, or the 6½" diameter Concentric coil [size=small](those were labeled Blue Max 600 or5.3 Black Max or 5.3 BullsEye)[/size] which measures about 6½" across the center of the coil.
The so-called 5" DD for the Teknetics measures just slightly more than 4½" and worked great on my Omega and Euro-Tek Pro. Right now I have it in use on the Fisher F-19. I like the Euro-Tek Pro's dual-range Volume control and ran it at '13' so Iron was a low Volume and non-ferrous targets were louder Volume. That's the main feature I like about the new Fisher F-19, it has that Ferrous/Non-Ferrous Volume adjustment that I set at '14' ... PLUS manual Ground Balance. That makes a lot of difference in the bad ground areas I hunt, but the Euro-Tek Pro worked fine for a lot of quick and easy coin hunting.
Monte