Pinched this from h++p://www.astra2d.com/malta.htm
Island of Gozo (NO ASTRA 2D) Panasonic TUDSB30 1.2m Channel Master fixed dish LNB Syntec 3000 Signal strength about 66%, signal quality 50-60%. Most channels received except ITV1 i.e. All Sky Sports and Movie channels, use a Satook alignment meter to keep it tuned in.
which concurs that 1.2m is too small for 2D.
What would be big enough? No-one seems to have posted a positive result but I would guess at something in the order of 4m or more. And even then it would be a suck-it-and-see job.