"of medium size" is a correct and usable phrase in written English.
It can be used to describe something that is neither too small nor too large. For example, "My family has a medium-sized dog."
At first it's of medium size and white; then it's larger and black.
One of the three halls, of medium size, is already in use.
"In this range," he added, "we feel ourselves to be a country of medium size".
"For African men, they need a woman of medium size -- I'll show you the size -- and shape.
The region has a large rural population, but there are also many towns of medium size.
This should be easy enough, especially if you stick with those of medium size, no more than four or five ounces each.
The park is one of hundreds of medium-size refuges throughout the city.
Pop has become a world of medium-size principalities with permeable borders.
Now it's a low-rise town of medium-size hotels, restaurants, bars and shops.
Aetna also wants to increase its share of medium-size customers and to sharply reduce Aetna-insured H.M.O.
Earlier this month, he listed 2,500 pounds of medium-size shrimp on Gofish at $10.50 a pound.
