I noticed this page comes high in SEO as I was searching for an answer myself. I found one fairly easily.
When embedding a tweet the first line will look something like this:
blockquote class=“twitter-tweet” lang=“en”>
Just add width=“x” after the lang component…
blockquote class=“twitter-tweet” lang=“en” width=“325”>
In fact there are many things you can add at this stage to modify the embed. Some are helpfully listed on this post by Wordpress: https://en.support.wordpress.com/twitter/twitter-embeds/
Hope this helps someone in the future.