まず、"How big is the meal?"「この料理の量はどれくらいですか？」
と、メニューを指さしながら聞いてみて下さい。
ちなみに
"How big is this?"
「どれくらいの量ですか？」
と聞いたり、
"Is this big enough for the two of us?"
「これは二人で食べても十分な量がありますか？」
と聞くのも良いかもしれませんが、
レストランによっては１人１オーダー制ですので、
そこはきちんと先に確認してくださいね。
If you are unsure if a dish would be big enough for sharing then you can simply ask 'how big is this?' or 'is this a big dish?' if you are wanting to explain that 2 of you will be eating it then you could say 'would this feed 2 of us?'
その料理が人とシェアできるだけの量なのか確認したいなら、次のように言えます。
'how big is this?'（これはどのくらいの量ですか）
'is this a big dish?'（この料理は量は多いですか）
もし二人で食べるなら、'would this feed 2 of us?'（二人分の量はありますか）と言えます。
When referring to a meal served at a restaurant, you may call it a "dish", "serving", a "plate" or a "portion." You may ask how big it is (1) or or you could ask about the size (2). You could also be more specific and ask if the portion is enough for one or two people (3).
When ordering food at a restaurant, it is important to know whether or not the meal will be big enough to fill you up.
We can ask about the size of the dish, (small, medium, large) to determine whether or not we feel the size is sufficient.
If the meal is not big enough we can ask them to upsize it or we can order something else on the menu.
If you were at a restaurant with your family or friends and you wanted to know the size of a certain meal, whether you are wanting to share as single dish or if you were wanting to save some for later on, then you could ask your server any of the above example sentences in order to extract the information you need about the size of the meal.
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