It depends on the organization and the type of projects it does. Could also be a function of the organization.
For example the value (cost) of a small project in the Oil & Gas industry could be much bigger than a major IT project or marketing project.
A small project for major organization could be a large project for a small organization.
So basically size is relevant with an organizational context and cannot be generic across all industries and companies.
It is also possible that some organizations use complexity factor instead of size or dollar value to define the size of a project.