Any matrix QM with restrictions on input cells displays an error indicator when the restrictions are violated, so the issue in the given question can only be about indicating the excess of the input information length. Since there is no option for input information length in the Constrain input mechanism, the problem can be solved through the Conditional format mechanism. To do this, you can use the conditional formatting expression:
Len(range) > Limit.
range – the range of cells with restrictions;
Limit – the number of characters of information exceeding which the cell indicator activates.