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That would depend on where the same would be opened (webmail or apps, clients), but most of them should resize your image to make it fit in.
My personal suggestion would be to use an image that is (in terms of maximum size) of the same width as your responses table.Now depending on what you are after and what kind of the image it is (logo, or if it should take the full width), I would suggest going for 320 or 420px width (but this is my personal opinion on the same).The best is however to do few tests. You can do that by using the Test button in your email - the email is then always sent to the email associated to your account.Once you have it just as you like it, you can submit the form itself to see how the actual submission would look as well.