The phrase "large vegetable" is correct and usable in written English.
You could use it to describe the size of a vegetable, for example, "I picked a large vegetable from the farmers' market."
"That is a large vegetable.
With a large vegetable garden steps from your bedroom window, you can easily improve your health.
There is a large vegetable garden tended by congregants who donate the produce to soup kitchens and needy families.
When I arrived at the house the first thing I noticed was a large vegetable garden outside.
They still have a few sheep, hens and a large vegetable garden — enough to maintain the farm feel.
Chimborazo maintained a large vegetable garden on a nearby farm, as well as herds of goats and cows.
The production region is often the most satisfactory location for prepackaging, especially when a packaging centre serves a large vegetable-growing area.
It comes with a hand guard with extra-long prongs, for gripping large vegetables.
At the painting's center is a heap of large vegetables -- the country come triumphantly to the city.
They created a vegetable patch outside their caravan and, whether by divine intervention or more prosaic influences, exceptionally large vegetables started to appear.
Place the vegetables on a cutting board and cut all large vegetables into bite-size pieces.
