Generally what you need in order to control the Paypal button size is to manually add to the image field the measurements using CSS code which is inserted directly as inline CSS, instead of a separate style sheet. Also you should delete the black background from the button and the frame.
The part of the code that controls the size should look like this:
"... The safer, easier way to pay online." style="width:96px; height:21px; background: none; border: none;">...."
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