Howdy there,..
If you want them the same size you can either keep all the post as the Format – Standard(with the pin icon,
or when you create a different format post, use the Insert More Tag (the 12th button on the post editing page). You can add this More Tag in a place appropriate for keeping them the same.
There probably are other ways, but these are the easiest ways that I would know to do without going into the website’s code.