Depends. Offset dishes are usually under 4ft across and intended for Ku (11700 -12000 Mhz)
Most common sizes for Ku FTA start around 30 inch up to 48 inch.
Prime focus, the BUDs, are 6 ft and up in diameter and intended for C band (3700 - 4200 Mhz)
Most prized are the 10 ft. Some use smaller, some use bigger.
Under 10 ft size can run into problems with interference from adjacent satellites.
As do offset dishes, on Ku, under 36 inch.
Many prime focus dishes also perform well on Ku with precise set up and aiming.
Some are Mini or Micro BUD(ing) - seeing what will come in on C band using a Ku offset dish.
Definitely not recommended as an entry point to the hobby.
As for all dishes performance- BIGGER - is better.