This array formula entered with the key combo of
CTRL,SHIFT,ENTER will tell you how long the longest entry
is in a range of cells:
=MAX(LEN(A1:A100))
If you want to identify that entry, you could use
conditional formatting:
Select the range A1:A100
Goto Format>Conditional Formatting
Formula is: =LEN(A1)=MAX(LEN(A$1:A$100))
Click the Format button and select a background fill color
OK out