Well I was reading on the pinboard theme support page and someone asked the same question. Unfortunately the answer that was given by Daniel Tara
was “Videos are made to be responsive, that’s why they behave like this. There’s no way to control their dimensions.”
But then Alexander said “Viper’s Video Quicktags is the plugin you need.
I tried it myself with Vimeo and youtube and it works like a charm.”
I havent tried it yet myself. I was just thanking I would give up on pinboard bc if I cant chance the video size that would be a deal breaker for me. I also read somewhere before that the only way was to edit the theme and that proved to be more than I could figure out. Let me know if it works.