I can't control its dimensions, and styles too.
If your template styles change infowin content styles, you can try to override it by adding styles for tag in map div (map div can has default CSS name, but you can set your own by map property).
You can also add all your infowin content into Addition HTML text field and apply any styles (I just display it without any changes). Probably, if you add your text into div, and apply to if some dimensions, infowin will use it for its size. Try it (but I don't check it)
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