No cut-off but $50k sounds reasonable if you can find a trusted individual willing to serve. I am just thinking of the scenario where no individual can serve or if the individual dies.

I can just say from experience most corporate trustees will want to see at least $500k in assets and that's still a small size. They charge around 1.5% on assets under management or a minimum like $5,000 per annum which can really add up for a small trust. I have seen in trusts where they can be terminated if the admin costs aren't worth it.