Tricky question jxk
You want answers concerning unregulated operations but only answers that "reference some legal requirements.."
A non-licensed airstrip does not 'require' anything by way of protection as set out in ICAO Annex 14 or CAP168 Aerodrome Licensing. CAP428 was introduced to offer GA a guide based on best practice derived from regulatory publications such as CAP168.
You can have a strip width as wide as you like, but woe betide you if you upset a NIMBY and go beyond whatever planning rule you operate under.
Without knowing what field you have in mind, what obstacles are close by and the biggest a/c liable to operate out of said strip it's all a bit hypothetical.
There are a lot of experts around and creating a safe environment for permitted ops will only enhance your position, but I suggest squaring away any planning issues first before creating clear and graded areas around the runway.
Good lick anway.
Sir George Cayley