The script console input (available on yourJenkins/script) shows the drag symbol in the lower right bottom of the box, similar to how it does for other input boxes:

However, you can't drag it smaller or larger, as opposed to any other text input, e.g. project descriptions.
I didn't take a closer look at it yet, but this seems to be down to the adjuncts in scriptConsole.jelly, importing a couple of presets from stapler-adjunct-codemirror.

I'm not sure whether this is needed still, because f:textarea supports specific Codemirror language support out of the box via "codemirror-mode", according to the documentation.