Most dish size charts provide the minimum size based of a full transponder at average modulation, power and FEC.
Narrow bandwidth, more aggressive modulations, high FEC and lower link budget calculations require larger and higher efficiency reflectors. Rejection of adjacent and terrestrial interference also increases the target transponder's CNR.
If a portion of the C-band spectrum.is auctioned off, expect more aggressive modulations and higher FEC rates. Certainly won't be sorry to have a larger and more efficient reflector and feed.