It appears you currently have the field set to return the image object, if you scroll down to the bottom of the page you linked you’ll see an example object:
Array
(
[id] => 540
[alt] => A Movie
Specifying a size when displaying an image associated with a taxonomy through ACF => Movie Poster: UP
=> sweet image
Specifying a size when displaying an image associated with a taxonomy through ACF => a man and a baloon
[url] => http://localhost:8888/acf/wp-content/uploads/2012/05/up.jpg
[sizes] => Array
(
[thumbnail] => http://localhost:8888/acf/wp-content/uploads/2012/05/up-150x150.jpg
[medium] => http://localhost:8888/acf/wp-content/uploads/2012/05/up-300x119.jpg
[large] => http://localhost:8888/acf/wp-content/uploads/2012/05/up.jpg
[post-thumbnail] => http://localhost:8888/acf/wp-content/uploads/2012/05/up.jpg
[large-feature] => http://localhost:8888/acf/wp-content/uploads/2012/05/up.jpg
[small-feature] => http://localhost:8888/acf/wp-content/uploads/2012/05/up-500x199.jpg
)
)
so for a specific size you want to replace $taxonomy_image['url']
with $taxonomy_image['sizes']['thumbnail']
, where thumbnail
is the desired size key.