Old ALV GRID: REUSSE_ALV_GRID_DISPLAY

I am modifying an old ALV report. In the structure used by the ALV Grid, I have a field defined as a STRING. The field catalog (i.e.: IT_FIELDCAT) is not being filled. The program uses the structure definition as default.

The problem is that the data for the field defined as STRING gets truncated when it goes beyond a certain length.

Is there a way to prevent that?