Fisher makes five coils sizes/types that fit the Fisher F5:
5" round DD
5x10" DD
7" x 11" DD
8" round concentric (comes off Teknetics machines)
10" eliptical (stock coil) concentric.
There are after market coil mfg's that also make coils for the F5. They include:
Deteck Ultimate 13" round DD
SEF 12" x 10" Excelerator Butterfly DD.
NEL Sharp - 5" DD
NEL Sharpshooter - 5.5" x 9.5" DD
NEL Hunter - 12.5" x 8.5" DD
NEL Tornado - 12" x 13" DD
NEL Storm - 13" x 14" DD
So there are lots of coils sizes available for the Fisher F5.
Regards your question about depth, yes, the Fisher 11" DD coil will give you more depth as well as better ground coverage. Probably at least 3" better in most dirt as compared to the stock coil,
IF you can maxamize the detector settings to use the bigger coil.
As the coils get bigger, attainable depth ( actuall depth in the ground ) depends on a lot of a factors. Soil mineralization, and EMI will all play a factor into how much more depth a bigger coil will actually achieve in the ground. Bigger coils can be more succeptable to EMI noise, and mineralizations. So site conditions place more limitations on how hot you can run your detector with bigger coils as compared to the smaller sizes. You will find you can run your F5 with hotter settings using the 5" and 5 x10" DD than you can running the stock coil in a lot of site conditions. You will find that you can run the stock coil with hotter settings than you can the 11" DD in many site conditions. But if the site conditions allow it, you can get some good performance out of the bigger coils.
I have all the Fisher made coils for my F5 but mostly use the stock coil and the 11" DD. The 5x10 is on my Eurotek Pro. I've been eyeballing the NEL coils but haven't pulled the trigger on one yet

The European websites have good reviews on those aftermarket coils.
HH
Mike