You can change
<input> of pagesize to
<select> with this plugin.
In Roundcube, pagesize can be set to any value. If the user sets an extremely large value, the mail server may be heavily loaded. In order to solve this problem, it is necessary to limit the values that can be set.
Using this plugin, you can not only change
<select>, but also limit on the server side so that only the value set in
config.inc.php can be posted.
All you have to do is
composer install this repo and edit
$config['plugins'] = ['archive', 'zipdownload', 'select_pagesize']; // Add array of option values. Values are int. $config['pagesize_options'] = [10, 20, 50];