[TYPO3] Setting default value for BE-form field "imagewidth" in TCEFORM

[Jens Beckmann]

Hi,

I want to set a default value that should be used for imagewidth to have 
any image rendered with e.g. 300px width. How do I manage this?

I think I will do it inside Page TSconfig. But there is nothing like:

    TCEFORM.tt_content.imagewidth= 300        or
    TCEFORM.tt_content.imagewidth.value = 300

Any hints? Thank you
Jens