THURSDAY night is gallery trolling night in Chelsea, so it was not surprising last week to find a crowd upstairs at the Max Lang gallery, examining the early works of the interior designer Scott Sanders. Under the proud eye of the curator, Shirley Sanders, the crowd examined drawings like ''Self-Portrait With Rosy Cheeks,'' a crayon on paper, from 1969, which showed a grinning little boy in dark glasses who looked eerily like the 47-year-old Mr. Sanders looks today; ''Colonial Home,'' a ballpoint pen on paper, from 1977; and ''Mega Mansion,'' a marker-pen drawing on paper, from 1975, of a great house set on a hilly landscape. [ABSTRACT FROM PUBLISHER]