Hi RFerwerda,

There is no general option for that.
What you can do is to go to [Edit] - [Select All] (Or hit its shortcut key [Ctrl + A]) on Activity diagram to select all the model elements (you can exclude non-actions if you want) and open its pop-up menu and check [Auto Resize]. (How you do with each individual action.) This applies to all the selected actions at once.

Thanks,
SJ