No. You can only change the properties (Thickness, Color, etc.) of your signature AFTER creating it.
Once you've created your signature, you can
- Tap the handwritten signature icon (the right-most icon) in the toolbar.
- Tap where you want to add your signature. You will see the popup menu showing "Add Signature" and "Edit Saved Signature".
- Tap Edit Saved Signature to edit it. You can change the thickness or color of your signature here.
Additionally, when you tap a newly added signature, you will get a blue rectangle outline with four grabber circles like this.
You can drag one of the grabber to change the overall size of your signature as well.