How do you say, is supposed to be, as in According to experts supper is supposed to be the smallest meal.
Experts say supper is supposed to be the smallest meal.
2 Answers
You can't translate that literally...
is supposed to be... would roughly translate as... "se supone que debe ser", "Se supone que es"
but in this case...
Los expertos dicen que la cena debería ser la comida más pequeña.
If you don't need the 'expert' part, only 'they say' as is often used in English, this is fine:
"Dicen que va a llover",
"They say it's going to rain.