Collectors do not grade inclusions by size. They grade by how much of the animal survived, how clearly it can be seen, and how honestly the piece has been prepared.
Completeness
A whole insect with legs and antennae intact is rare, because struggling in resin usually tears something off. A complete specimen at four millimetres is worth more than a partial one at twelve.
Depth and clarity
Resin is not glass. Every millimetre between your eye and the inclusion scatters light, and a specimen sitting deep in cloudy material may be scientifically interesting and visually useless. Position near a polished face, in clear surrounding resin, is what you are paying for.
Preparation
Many specimens are polished thin, sometimes shaped as a lens, to open the view. This is normal and skilled work. It should also be disclosed, because it changes both the fragility and the value of the stone.
What you will actually find
Most inclusions are midges, flies and ants — the insects that were simply most numerous. A spider is a good find. A vertebrate is a headline, and almost always something else.