Custom image size wrong in PayPal Donations Button

I’m trying to use the custom image in the Accept PayPal Donations plugin. I’m able to insert the correct image to use as the button, but when adding it into the widget sidebar it doesn’t fit properly. I’ve tried changing the size of the image through trial and error and was able to get it to fit, but this only worked on my laptop. When I then viewed it on a tablet or mobile the size is incorrect. Are there any settings that will ensure the button image fits correctly across all of the widget locations?

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