Comment # 1 on bug 964400 from Dominique Leuenberger

I want your screen! :)

The issue seems rather simple:
src/org.gnome.Terminal.gschema.xml:

    <key name="default-size-columns" type="i">
      <range min="16" max="256" />
      <default>80</default>
      <summary>Default number of columns</summary>
      <description>Number of columns in newly created terminal windows. Has no
effect if use_custom_default_size is not enabled.</description>
    </key>

so the max we accept there is 256