it is possible to use a non-standard image size.
as far i can see you use the correct variables.
$image = get_field('your_image_field');
$size = 'name_of_size';
$url = $image['sizes'][ $size ];
$width = $image['sizes'][ $size . '-width' ];
$height = $image['sizes'][ $size . '-height' ];
just to get sure that your preferred size exist, what is inside your image variable?:
$image = get_field('your_image_field');
echo '<pre>';
echo htmlspecialchars(print_r($image,true));
echo '</pre>';