well your question is a bit confusing because you say you had 192px “Thumbnails” assuming you mean the_post_thumbnail then the actual original image size that you uploaded originally could and would most likely be larger, in which case the regenerate thumbnail plugin would work for you.
when you say you can not access featured images inside the header as it not in the loop this is also confusing because if it is already using smaller featured images this is misleading.
that being said, you could potentially move the portion of the post that contains the featured image out of the header and into the single page template.
Sorry this should probably be a comment, in short if you can provide more info a solution should be available.