I suspect that the limit is the same as for a single cell:
Extract from Specifications:
32,767 characters. Only 1,024 display in a cell; all 32,767 display in
the formula bar.
...
And the specs are as accurate and complete as senior MSFT execs testimony in the
last antitrust trial.
If the cell evaluates to text with newline characters every 100-200 characters,
the typesize is small enough, and the column width and row height large enough,
Excel happily displays over 10,000 characters in a cell. Indeed, with column A's
width set to 150, rows 1:3s' height set to 409, cells A1:A3 merged, font set to
Arial with typesize 6, the formula
=REPT(REPT("#",254)&CHAR(10),128)&REPT("_",130)
happily displays all 32,767 characters that Excel is capable of displaying or
evaluating to. So much for specs. As for those who spout them uncritically, ...
Specs are only as good/accurate as the people who write them care about making
them. Based on evidence, Microsoft doesn't seem to give a damn about 'em.