I have been having real trouble controlling the size of my image in MailChimp when it arrives from an RSS feed. It seems to use the size from the feed (WordPress), and having a class on the surrounding table doesn't work.
You cannot control the size of the image in CSS or HTML. This is not to do with MailChimp, but to do with Outlook. Outlook will use the original width of the image (not the width as you see it on the website but the width if you view the image alone).
On the whole a website image will be much larger than you want in an email as emails are commonly 640px wide at most.
Whereas websites are 1200px or wider. In these situations, in Outlook the image will be huge and totally blowout the layout of the email even if you constrain the table width.
The only way to change that is to set the width in the RSS feed as a HTML element e.g.
<image src="image.jpg" width="300" height="auto">.
or
<media:content xmlns:media="https://video.search.yahoo.com/mrss" medium="image" type="image/jpeg" height="auto" width="280" url="https://www.YOURWEBSITE.com/images/YOURIMAGE.jpg">
<media:description><![CDATA[YOUR DESCRIPTION OF YOUR IMAGE]]>
</media:description>
</media:content>
I believe in other email clients the issue is not a problem and you can use
img {width: 100%;max-width:100%;}
but most people will need to take Outlook into account (I assume)