The phrase "a usual portion" is correct and usable in written English.
It can be used to refer to a typical or standard amount of something, often in contexts related to food or servings.
Example: "At the restaurant, I ordered a usual portion of pasta, which always satisfies my hunger."
Alternatives: "a standard serving" or "a typical amount".
Each food and beverage item had a specified usual portion size as done in the Willett FFQ.
Humana has moved half of its commercial customers to a four-tier copay scheme, charging patients a higher than usual portion of the price for big-ticket biotech drugs like human growth hormone; it is unveiling an even more radical scheme that gives consumers set "allowances" for each category of drugs this year.
When study participants were asked to compare the portion size of various sandwiches (presented as a single unit) to the usual portion of the same food, they rated the portions of the larger sandwiches as larger than their usual portion of sandwich (Rolls, Roe, Meengs, et al., 2004).
Participants also rated how the displayed portion compared to their habitual portion using a 100 mm VAS preceded by the question "how does this serving compare to your usual portion of X food/drink?" ("a lot smaller" to "a lot larger") (Kral, 2006).
VAS ratings for habitual portion size comparisons were summarised with means and 95% CI, where a 50 mm value was considered to signify 'same size as usual portion'.
For each food item, frequency of consumption (categories ranging from never to once per day or ≤ 5 times per day, depending on the item), and usual portion size (small, medium, or large relative to a given standard medium portion) were assessed for the previous year (i.e., 1999).
We drank more than the usual portion of golden wine, like liquid sun caught in a glass.
A healthy housing sector could boost GDP by more than $400bn, based on housing's usual portion of the overall economy.
Usual portion sizes will be obtained in household measures and with the use of photographic aids.
