I have a dual band feed from a 7.5 mesh and it works very well. It is actually two LNBs on a dual band feedhorn, not a single LNB for both. I do not have anything for HD FTA (yet) but from what I've read, 10 foot is about the accepted minimum standard size for the DVB S2 signals.
It is a bit confusing until you think it thru, but as has been posted, you have to set up 2 config pages for each orbital position that has both C and Ku signals that you want to see. There are exceptions. Some have no content useful to FTA on C band, same goes for others for Ku signals. For details you will need an up to date DVB signal list to build your configuration.
A legacy C band dish will likely have a servo driven polarotor for polarity control. Some prefer the orthogonal, two state LNBF like the one mentioned above. All DVB FTA receivers can interface to, and control polarity of those type of LNBFs. I prefer the polarotor design for it's fine tuning capabilities, but the drawback is that (with a few exceptions of certain models) a DVB box cannot directly operate the servo. The most popular method of controlling the servo is with a legacy analog C band receiver. There have been some homebrew solutions, but nothing else is commercially available AFAIK.
A 920/922 4DTV receiver will do analog and DCII (Digicipher 2, Motorola's version of digital satellite broadcasts) and is polarotor control equiped as well.