What would be a reasonable set of thresholds for flood severity classes?
One existing classification for return period events is minor and major flows, which basically classifies floods into those that the storm network (pipes and small conveyance network) is designed to handle, typically in the 2-10 year event range, and those more major floods which will overwhelm the stormwater infrastructure and are mainly conveyed by overland topography and larger flow channels/watercourses (>10 year event). The definition of major and minor return periods depends on the level of design in your are of study, but that is one classification. This distinction in major/minor flows is common and you can find lots of resources, one example is in the Ontario Drainage Manual. 

Given that you want a small/medium/large classification, I would put minor flows (say 5 year and smaller, based on the study area) into the small category, everything above a minor flow to the 100 year event into the medium category, and everything 100 year or larger (i.e. 100 year and regulatory event) into the large flood category. The 100 year as you mentioned is a very common standard for a large flood event. The Regulatory event is typically the largest observed flood on record or similar estimate for a larger than 100 year flood event.

Hope that helps!