First, that's not ok to put inline styles, you are doing it wrong. You should inspect the header cell and find the class that gives the width or the space, then you just needed to add some sort of custom class or even customize the class directly on the parent class that gives the width.
But, if you want that to work, It should be style instead of width and where you have "70px;" it should be "width:70px;"
So on the property input, you put style and on the value, you can put "min-width:70px;"
Check this post
https://www.outsystems.com/forums/discussion/69758/resize-the-width-of-the-cell-tablerecord/
Like this

Kind Regards,
Márcio