I would say that generally speaking, I'm against anything over 55lbs. It's clear that FAA views those differently, and quite honestly I think those look more like something that should be individually registered, airworthiness checked, and inspected periodically (by someone in FAA).

As for the rest, I think we should be looking at total energy. That way it automatically accounts for mass, velocity, and altitude. Beyond that, I think limits need to based on the airspace. I personally believe there should be be altitude separation between majority of manned aircraft and unmanned. I'm skeptical of spotters as a mitigation. Why? AMA's own video demonstrating spotters show both operator and spotter tracking the aircraft. An EFFECTIVE spotter should be looking 360 around the site, NOT be a second set of eyes looking at the same airspace as the operator. Without visual acuity verification and hearing checks, my confidence in spotters goes way down. So that means in class G, it's 400 AGL. However, with altitude reporting telemetry and alarm on transmitter, I could support recreational sUAS limited to no higher than 100 feet below floor of class E at the site. In controlled airspace, I support whatever ATO agrees. Additionally, I advocate a ban on recreational sUAS inside the lateral limits of Military Training Routes during published hours of operation.

I don't want to give a hard and fast size, but rather limit by TE, flying site dimensions, altitude limits per above, and flight path. Idea being that at any point in the path, based on the current velocity vector, a complete loss of control would result in sUAS impacting inside lateral limits of FRIA and no closer to 100 feet from any non-participant (FAA's definition of participant) inside the FRIA.