You can add this code in functions.php

function ast_excerpt_length( $length ) {
   return 120;
}
add_filter( 'excerpt_length', 'ast_excerpt_length' );

By default WordPress outputs 55 words for excerpts. The code above increases it to 120 words. You can change the number as you like. The box should expand automatically.

Add this on the Custom CSS tab of the Asteroid Options. Or you could add it on a style.css if you are using a child theme.

.entry-excerpt {
    text-align: justify;
}

No, that just justifies the text on the excerpt.
You can add this code on Custom CSS and make the thumbnails a bit bigger but not much.

.blog-view .entry-thumbnail {
    height: auto;
    width: auto;
}