Hi, I finally figure it out. (In Moodle 2.7)
These post helped me: https://moodle.org/mod/forum/discuss.php?d=231488
So, in the file /moodle/mod/assign/style.css at the end I write the following lines:
/* Expand the file submission column width - Header */
.gradingtable tr th.header.c8 {
min-width: 250px;
}
/* Expand the file submission column width - Cell */
.gradingtable tr td.cell.c8 {
min-width: 250px;
}