Since the most useful default size will probably very much depend on the actual data that is displayed in a BPMN diagram, I think the most sensible solution is to create a user-defined palette section with templates that match your requirements:
- Create a user-defined palette section (see "Edit" -> "Manage Palette").
- Drag a BPMN task activity from the default BPMN palette into an editor tab.
- Adjust the size (and every other property) of the activity to meet your requirements.
- Right-click the activity to open its context menu and choose "Add to Palette".
- Use the custom template when creating new activities.
The above approach should work fairly well, if you need to adjust only a few templates. If you want to adjust (almost) all templates in a given palette section, I propose the following approach:
- Right-click the header of the BPMN palette section (or whatever section contains the templates you want to adjust) to open its context menu and choose "Convert to Document".
- Adjust all the templates in the new editor tab to meet your requirements.
- Create a new user-defined palette section, but be sure to choose "Adopt from Document" to automatically populate the new section with all the elements from the currently active editor tab.