Hi,
There are currently only 3 widget sizes available as explained here: https://developers.cloudflare.com/turnstile/get-started/client-side-rendering/#widget-size
You can choose between these 3 options in the plugins advanced settings, in the latest version of the plugin.
I can just second this. The same question also here -> https://wordpress.org/support/topic/make-responsiveness-better/
on older mobiles with 320px width the 300 min-width from the horizontal view doesn´t help. The vertical squared view looks, mmh, less optimal on wide screens.
Why cloudflare doesn´t let us decide whats the perfect max width should be? Or why doesn´t cloudflare not going down to, lets say, 280 px on horizontal view? That would be enough for some padding and still not messing up the layout on mobiles.
EDIT: setting the challenge to flex and the iframe-container to overflow: hidden at least prevent a complete ugliness 😉
