It’s hard to say, it really just depends on the image. The height that you want (in addition to the width) is important when adding images. I can’t think of a “One size fits all” recommendation since people will view your site from different browser sizes. I would try and start out with the largest size you can and see how it looks on a large monitor (if one is available), then view on a laptop, and then on a mobile phone.
If you upload something just let me know and I can take a look to let you know how it looks on my end. I have an iMac, Macbook, and iPhone if you need me to check for you.
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