altScreen resource

From The x3270 Wiki
Type Default Option
string -altscreen

Defines a character string that will be output to switch the terminal from 80-column mode to 132-column mode. Within the string, the sequence \E is translated to the ASCII ESC character (U+001B).

When the 3270 model is 5, and this resource and the defScreen resource are defined, c3270 will automatically send this string to change the terminal size when the host switches from the default (24x80) screen to the alternate (27x132) screen.