Sure, if you wish to publish only the featured image, you can do so by adding this PHP snippet to [your theme’s functions.php
file or where you normally add these tweaks]:
add_filter( 'share_on_mastodon_attached_images', '__return_false' );
See also: https://jan.boddez.net/wordpress/share-on-mastodon#share_on_mastodon_attached_images
The next version of the plugin, which I hope can be released in a number of days, will have a “real” option for this, so you might want to wait for that instead.
Re: the suboption you mention, I don’t see that happening very soon, sorry.
The way it works right now, the plugin will always upload the “large” thumbnail, or, if it can’t find one (this sometimes happens when you use a CDN), the original image file. (Either way, Mastodon will size down/compress these as it sees fit.) Do you use a plugin to “manually” tweak/crop each thumbnail size, then? Because if not, I fail to see why you would want to upload a larger (or even smaller) size …