in size
Comparing dimensions
Use “in size” when discussing differences or changes.
The files vary in size considerably.
“in size” or “of size”
“In size” is common when comparing dimensions or amounts. “Of size” is uncommon alone but natural in phrases such as “of similar size” or “of a certain size.”
Quick answer
How do these phrases differ?
Use “in size” when comparing or measuring changes. Use “of” before “size” only in descriptive phrases such as “of medium size” or “of equal size.”
The boxes differ in size and weight.
The two rooms are of similar size.
The two rooms are of similar size.
Choose the pattern by meaning
These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.
of + size phrase
Describing dimensions
Use “of” with an adjective or determiner before “size.”
They bought a table of medium size.
size of + noun
Naming what is measured
Use “size of” to identify an object's dimensions.
We checked the size of the screen before ordering.
See the difference in context
The same subject can take different constructions because each sentence answers a different question.
Construction
Example
Meaning
in size
The planets differ in size.
Used for comparison or variation.
of size
The samples are of equal size.
Used with descriptive modifiers such as “equal” or “similar.”
size of
The size of the apartment is ideal.
Refers to the dimensions of something.
Common mistakes
Some combinations appear in real text but do not mean what learners often assume.
Using “of size” without a modifier
English usually needs an adjective or determiner after “of.”
✕ The bags are of size. ✓ The bags are of similar size.Confusing comparison and description
Use “in size” for differences and “of size” for qualities.
✕ The shoes differ of size. ✓ The shoes differ in size.Quick practice
Choose the expression that matches the intended meaning.
1. The two images differ ___ size.
2. The containers are ___ equal size.
3. We measured the size ___ the package.
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Frequently asked questions
Is “in size” correct?
Yes. It is common in comparisons such as “differ in size.”
Can I say “of size”?
Yes, but usually with a modifier, such as “of similar size” or “of large size.”
What is the difference between “in size” and “of size”?
“In size” focuses on comparison, while “of size” describes a quality or category.
Related preposition guides
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