Add pytest markers indicating relative size of tests

This issue focuses on enabling the use of [pytest markers](https://docs.pytest.org/en/7.1.x/example/markers.html) to allow for more granular testing during development of this repo, for example `poetry run pytest -m small` to run "small" tests. Using markers could also allow for dagger based testing to use these markers for reproducible testing results, for example `dagger do test 3.9 pytest small`. As a start towards this work, `small`, `medium`, and `large` could be used for sizes (using only very rough estimates). These markers would allow for quicker results when developing specific aspects of this project.

Start by locating the repository's pytest configuration and test suite, then review how tests are currently invoked with Poetry. Add support for small, medium, and large markers with rough test-size assignments, and verify that marker-based pytest commands produce the intended subsets of tests.

Written by the indexing model from the issue text.