I was using thumbnails of 192 pixels square but want to change the size to 235 pixels, the Regenerate Thumbnail plugin does not upsize so since I want thumbnails above 200 pixels to use the Twitter Card with Large Image. I also don't want to manually replace all my old thumbnails (Featured Images) with larger ones, but I thought I could use some code to detect newer larger Featured Images which I plan to use, but I can not access the Featured Image inside the Header as it is not in the Loop correct?
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.