A ~500-km-sized object has been discovered (on old images) in the far reaches of the solar system - the orbit of #2017OF201 has a semi-major axis of 880 au and an eccentricity of 0.95 and thus a perihelion at 45 au: https://minorplanetcenter.net/mpec/K25/K25K47.html and https://groups.io/g/mpml/topic/113234338#msg40620 (and see https://www.minorplanetcenter.net/iau/Sizes.html for how to covert the absolute brightness H - which is 3.55 - into a diameter).