Hi there,
Not sure of any specific metrics in terms of A/B testing but think that in the right context gifs can make a marketing email stand out and be more enjoyable to consume. I wouldn’t use gifs in one-to-one sales email as think that is best to be plain text. But for marketing email, newsletters etc, and the right audience, then sure why not. Also Canva has introduced a gif maker so you can create nice on-brand gifs easily.
There are of course limitations including the size issue you mention. The smaller the better of course. Also people with older versions of Outlook won’t get to see the gif by default.
As for resizer, I normally just use one of the web ones - like Ezgif or similar. For Mac users there is this little app you can download: ImageOptim — better Save for Web
Happy giffing