Your grid row and column dimensions are set under the format tab of the actual grid question. These will squeeze the rows and column codes and other specify box into the grid according to your format settings.
The maximum number of characters are set by clicking on the other specify code and then click the "Settings" box which is under the "Respondent Specify" tick box. The options available are the maximum number of characters for the other specify comment, required response, multiple lines to display the comment box, and the dimensions of the comment box.
Good luck.