How to specify width and height in the_content for image attachments

I believe this is what your looking for. The complete code can be found in twentyten’s loop-attachment.php template. Look for the <p class="attachment"> tag. There are two filters there. One for width and the other for height (currently set to 900px for both).

<?php if ( wp_attachment_is_image() ) :
$attachments = array_values( get_children( array( 'post_parent' => $post->post_parent, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC', 'orderby' => 'menu_order ID' ) ) );
foreach ( $attachments as $k => $attachment ) {
    if ( $attachment->ID == $post->ID )
        break;
}
$k++;
// If there is more than 1 image attachment in a gallery
if ( count( $attachments ) > 1 ) {
    if ( isset( $attachments[ $k ] ) )
        // get the URL of the next image attachment
        $next_attachment_url = get_attachment_link( $attachments[ $k ]->ID );
    else
        // or get the URL of the first image attachment
        $next_attachment_url = get_attachment_link( $attachments[ 0 ]->ID );
} else {
    // or, if there's only 1 image attachment, get the URL of the image
    $next_attachment_url = wp_get_attachment_url();
}
?>
                    <p class="attachment"><a href="https://wordpress.stackexchange.com/questions/15801/<?php echo $next_attachment_url; ?>" title="<?php echo esc_attr( get_the_title() ); ?>" rel="attachment"><?php
                        $attachment_width  = apply_filters( 'twentyten_attachment_size', 900 );
                        $attachment_height = apply_filters( 'twentyten_attachment_height', 900 );
                        echo wp_get_attachment_image( $post->ID, array( $attachment_width, $attachment_height ) ); // filterable image width with, essentially, no limit for image height.
                    ?></a></p>

                    <div id="nav-below" class="navigation">
                        <div class="nav-previous"><?php previous_image_link( false ); ?></div>
                        <div class="nav-next"><?php next_image_link( false ); ?></div>
                    </div><!-- #nav-below -->
<?php else : ?>
                    <a href="<?php echo wp_get_attachment_url(); ?>" title="<?php echo esc_attr( get_the_title() ); ?>" rel="attachment"><?php echo basename( get_permalink() ); ?></a>