There’s some way to add a wrapper around posted images?

I need to add a wrapper (I’ll use <figure> in my example) around images posted through the editor, with a variable depending on the size image is being posted (thumbnail, medium, large, full), something like:

<figure class="$size">
    <a rel="attachment" href="https://wordpress.stackexchange.com/questions/11213/....>
         <img... />
    </a>
</figure>

I already solved the first step, adding the wrapper with a filter through image_send_to_editor, but I can”t find a way to get the image size so I can add it to the class.

Any clue?

The image_send_to_editor hook passes a whole range of parameters, not just the html. One of them is the size. To get it, hook your filter passing an 8 as the number of supported parameters:

add_filter('image_send_to_editor', 'my_filter_cb'), 10, 8);

and modify your function header to grab them all:

function my_filter_cb ($html, $id, $caption, $title, $align, $url, $size, $alt) {
    ...
}