Hi LB,
You are looking for a means of conditionally formatting the cell, with the condition being the length of its conrtent being greater than 255 characters.
Unfortunately, that condition isn't in the list of available rules that can trigger a conditional format change:
You could, however add a cell adjacent to the one containing the text, and use that to flag the cell when the limit is exceeded. Here's an example, using a maximum length of 10 characters to allow seeing the text and results in this post:
The formula in all cells of column B is: =LEN(A)
The result may be hidden by setting the default text colour in these cessl to white (I've used a light grey to allow the results to be seen). The conditional format can be set as shown below the table. If you do not want the actual count to be visible set the conditional text colour to match the conditional fill colour.
Note that you must confirm the contents of the column A cell before the calculation in column B will be done.
Regards,
Barry