L size; large size
Refers to the large size in clothing, drinks, or other items, equivalent to 'large' in English sizing.
このＴシャツのＬサイズをください。
Please give me this T-shirt in size L.
コーヒーはＬサイズでお願いします。
I'll have a large coffee, please.
From English 'L' (large) + サイズ (size).