I read that too, I think they mean when/if they get another satellite alongside the Rainbow satellite at 61.5, so it will have to be the above size to catch both birds! A lot of folks here use a 30" dish for VoOM, with great results, the only thing the bigger the dish the more they catch the wind, then instead of a rain fade problems you now have the dish moving slightly evey time the wind gets up with the same end result, picture in and out. I use a 30" dish right now for all satellites (on motor) then I am future proofed for all satellites forever, it does catch the wind but I can live with that as I know what it is : ) I try to keep all satellite dish as low as possible, that's where the pros put theirs, so that's good enough for me, now and again I have no choice but to put it on a roof, and the customer always knows best : )
The 72 degree slot that VOOM will use is KU FSS. FSS slots tend to be somewhat lower power and reside every two degrees--unlike DBS where slots are every nine degrees--thus a larger dish is required to distinguish between them. There are KU FSS slots at 70 and 74 degrees that would make a smaller dish for 72 degrees impossible.
Dish Network also uses a similar dish that can receive the 105 FSS slot (where they've put locals for smaller markets), plus the DBS slots at 110 and 119. Dish Network calls it the "Superdish" -- it is 24x34" or 21x36", depending on the version.
I can't imagine that the larger dish will be mandatory. My guess is they will use it for new services--like PPV, RSNs, and sports packages in HD--and possibly a new programming tier between basic and VaVaVoom. Obviously, they will need to keep any price increase on VaVaVoom to a minimum so customers who stick with the smaller dish don't end up paying much more for the same programming (since they wouldn't receive the new channels on the larger dish).
I posted some speculation on the larger dish right here.
If DBS was every 4.5 degrees would it still be able to have 18" dishes? If KU FSS was every 4 degrees or 8 degrees, etc. how small of a dish could it get to being? I remember that Voom was going to come out with a 13" dish but decided on the 18" dish and now 36".