"a standard size" is correct and commonly used in written English.
Example: The store offers a wide variety of products in a standard size, making it easy for customers to find what they need.
The recovered amphora was about three feet high, a standard size.
"Would you like to try them on?" "Is it a standard size?" "Um, yes".
Press this into a standard size pie dish using the back of a metal spoon.
Of course, I had to eliminate inexpensive ready-made frames, because my photo was not a standard size.
A standard size sham is $55, a Euro sham is $59 and a small decorative pillow is $45.
This site prints high-quality books in one-twelfth scale (a standard size for miniatures).
(The sink doesn't come with a disposal, but it accommodates a standard-size one).
If you prefer cremation, three MacBook Pro boxes will hold a standard-size adult.
Prices for a standard-size pillow range from $60 to $160, more in queen- or king-size.
A standard-size table is 22.5 inches high and 19.5 inches wide.
Distilling all the juice of her life into a standard-size container is, it seems, like trying to pour 10 gallons of fizzy water into a Dixie cup.
