Right-sizing logo image for signature block in mail

I cannot figure out how to re-size the logo file for my signature block for a gmail account. It is easy - and intuitive - to do in GMail, but the change doesn't transfer to Apple mail. I would like to be able to use Apple mail for all of my email accounts, but for some reason it won't play nice with google. And I see no way - including trying to change the image size in preview and then saving it and trying to attach it - to rightsize the logo. It just stays huge. The problem is the same whether on my iphone or macbook. Seems like it should be so easy...