This is actually not related to WooCommerce. The description area works just like any content area in WordPress. What you upload is what you get. So you have two options. Either you upload an image that is the correct size or you use the builtin WordPress native tools for image editing. You can get to the tools when you upload the image.
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splashingpixels.com on "[Plugin: WooCommerce - excelling eCommerce] Product Description Image Size"
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