No. A carbon atom does not have a fixed size.

And especially when you attach atoms together the spacing between them varies depending on the strength and type of the bond.

Additionally, molecules are constantly in motion (heat) - one of the motions is changing the size of the spacing between them.

But even if you froze it to absolute zero (which is impossible), it doesn't matter, the size is not determinate, it's only probabilistic.

So a real physical object can never have a fixed size.