How to change the height of featured image in Twenty Eleven child theme

If you want another size of the feature images in the Child theme than the parent theme you should add a functions.php file to the child theme and add specific functions for that theme.

From WordPress Codex about child theme functions.php

Unlike style.css, the functions.php of a child theme does not override
its counterpart from the parent. Instead, it is loaded in addition to
the parent’s functions.php. (Specifically, it is loaded right before
the parent’s file.)

So lets say you want the child themes feature image size to be 150x150px you just add add_image_size to the childs functions.php file.

An example when the parent already have support for post-thumbnails you just add the thumbnail size:

if ( function_exists( 'add_theme_support' ) ) {
    add_theme_support( 'post-thumbnails' );
    set_post_thumbnail_size( 150, 150 ); // default Post Thumbnail dimensions
}

If you want more sizes only on the child theme you just add:

if ( function_exists( 'add_image_size' ) ) { 
    add_image_size( 'category-thumb', 300, 9999 ); //300 pixels wide (and unlimited height)
    add_image_size( 'homepage-thumb', 220, 180, true ); //(cropped)
}

And then call it by:

<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'category-thumb' ); } ?>

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