If you are not sure which size button you need for your blouse or jacket, or if you want to use a different size than the pattern suggest, you can use a rule of thumb.
The size of the button will depend on the overlap (distance from center front till edge) you are using on your garment.
A guide for this is when the garment is buttoned up the distance from the edge till the button should be ½ of the diameter of the button or a tiny bit less.
You can measure this on your pattern.
A horizontal placed buttonhole starts always 1/8 or 3mm away from the center front marking line towards the edge.
You can adjust/alter the pattern for the overlay and underlay to match the size of buttons you want to use.
This rule of thumb is just a guide and not a obligation.
The link for the diagram is from Burda sewing technique book.