Short answer would be 245/70R16. Anything taller/wider will involve some level of wheel arch modification on a D1 with factory wheels.
However just trimming the return lip on the rear guard (ie. invisible mod) does open up some excellent alternatives (like 245/75R16) 
Lots of info on this forum about what sizes require what. Search is your friend
DiscoClax
'94 D1 3dr Aegean Blue - 300ci stroker RV8, 4HP24 & Compushift, usual bar-work, various APT gear, 235/85 M/Ts, 3deg arms, Detroit lockers, $$$$, etc.
'08 RRS TDV8 Rimini Red - 285/60R18 Falken AT3Ws, Rock slider-steps, APT full under-protection, Mitch Hitch, Tradesman rack, Traxide DBS, Gap IID
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