To get the address of the first long line, use:
=CELL("address",INDEX(A2:A2000,MATCH(MAX(LEN(A2:A2000)),LEN(A2:A2000),0)))
with an array record ( CTRL + SHIFT + ENTER ).
MAX(LEN(A2:A2000)) returns max lenMATCH(MAX(LEN(A2:A2000)),LEN(A2:A2000),0) finds the index of the first cell with max len in the range A2:A2000INDEX(A2:A2000,MATCH(...) returns a link to the first line with max lenCELL("address",INDEX(..)) retrieves an address from a cell reference

UPDATE:
as follows from the comments, OP uses the French version of excel, so the following formula works:
=CELLULE("adresse";INDEX(B1:B100;EQUIV(MAX(NBCAR(B1:B100));NBCAR(B1:B100);0)))
with array record
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