> ヒ?ェ窶敕披?堋キ窶堋ュ窶堙遺?堋「窶佚・窶堋ォ窶堋「ツ。the least biggest...? Does that work?
Sounds very strange/unnatural. You should use the opposite adjective(ヒ?ェ窶敕板渉ャ窶堋ウ窶堋「).
> Or should you just use the opposite adjective?
Instead of "the least biggest" construction, there's "ヒ?ェ窶敕 + adjective + negative affix" construction in Japanese grammar(ヒ?ェ窶敕披?佚・窶堋ォ窶堋ュ窶堙遺?堋「, ヒ?ェ窶敕藩??窶堋ュ窶堙遺?堋「, ヒ?ェ窶敕披?敕シ窶督。窶堋オ窶堋ュ窶堙遺?堋「, ヒ?ェ窶敕板広窶堋ュ窶堙遺?堋「, ヒ?ェ窶敕版?テー窶堋オ窶堋ュ窶堙遺?堋「......).
However, "ヒ?ェ窶敕 + adjective + negative affix" doesn't always mean "the least ---est". There's an alternative.
ヒ?ェ窶敕披?佚・窶堋ォ窶堋ュ窶堙遺?堋「--> 1)the least biggest, 2)anything/something but the biggest one
ヒ?ェ窶敕藩??窶堋ュ窶堙遺?堋「--> 1)the least cheapest, 2)anything/something but the cheapest one
ヒ?ェ窶敕板広窶堋ュ窶堙遺?堋「--> 1)the least widest, 2)anything/something but the widest one