First of all, because the noun "masa" is feminine, the adjective has to agree in gender, so "grea".
Secondly, "hard" can be translated to "greu" only when referring to difficulty (i.e. the opposite of "easy"). Describing a table, the Romanian "greu" can only refer to its weight, so it would mean "heavy".
It is not a correct English translation. And here's why: (1) great (of an extent, amount, or intensity considerably above the normal or average) => mare (literally, big / large / grand) (2) great (of ability, quality, or eminence considerably above the normal or average) => renumit (literally, famous / well-known / notable) (3) great (denoting the element of something that is the most important or the most worthy of consideration) => puternic (literally, powerful / potent / influential)
On the other hand, the Romanian tare is mostly used to denote something hard / tough / solid. Although, rather rarely, tare can also be used to describe something loud, for instance you can say "Dă sunetul mai tare" => "Make the sound get louder"
It is true that masa can mean both table, as well as meal in Romanian, however, in the original context, it would be really weird for a native Romanian to refer to the meal as being heavy, or even hard, for that matter.