/etc/php/${version}/cli/php.ini

Modify:

post_max_size upload_max_filesize

Ensure that post_max_size is considerably greater, since it also has to accommodate the accompanying HTTP headers and JSON.

${dotclear_base_path}/admin/blog_prefs.php

Edit: form::field('media_img_m_size',4,4,$blog_settings->system->media_img_m_size).'</p>'. to: form::field('media_img_m_size',4,4,$blog_settings->system->media_img_m_size).'</p>'.

to permit four-digit sizes

Restart blog PHP and FastCGI

Blog Settings -> Parameters

Increase 'Generated image sizes' as required