Button's 9-patch bitmap resolution

I want to make a 9-patch image for one button. What is the right resolution in pixel for that image?

Acording to this link
9-Patch Image – Designers Vs Developers - shubhayu.com

9-patch images can scale up but not scale down. Scaling down results in visual artifacts.
Always design for the smallest size possible. This will increase its re-usability.
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Do not use a 9-patch image that is bigger than your View.

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So, if a have a 40/40dip button, the right size in pixels for the 9-patch image must be defined having in mind a device with scale 1 in order to have the smallest size possible and so 40/40 pixels?