get_post_gallery() returns an array with ids of the gallery images and the urls of the thumbnail (150×150 by default). The ids are returned in a string with a comma separated list of ids. If you want to get a different image size, you will need to get it in the foreach loop using the ID of the image and a proper WordPress function, for example wp_get_attachment_image_src.
In wp_get_attachment_image_src you can specifiy which image size you want to get. It can be any of the core image size (thumbnail, medium, large or full) or any other custom image size previously registered. It can be also an array representing width and height, for example, array(150,150) (it must matcha the dimensions of a registered image size).
For example, to get the full image size:
$gallery = get_post_gallery( get_the_ID(), false );
$ids = explode( ",", $gallery['ids'] );
foreach( $ids as $id ) {
//Change "full" with the image size identifier you want to get
$src = wp_get_attachment_image_src( $id, "full" );
echo '<li class="slide-container">';
echo '<div class="image flt-left">';
echo '<a href=$src>';
echo '<img src="'.$src[0].'" width="'.$src[1].'" height="'.$src[2].'">';
echo '</a>';
echo '</div>';
echo '</li>';
}