Hello Simon,
1. There isn't an option to change the width of the filter field. It will show the proper width for the length of the filter title. You can change the filter title when editing the relationship and see how it works. Please check these screenshots https://imgur.com/a/MxohpEj
2. The text "Please enter 1 or more characters" is hard-coded in the select2 JS file https://github.com/wpmetabox/meta-box/blob/master/js/select2/i18n/en.js
inputTooShort:function(e){return"Please enter "+(e.minimum-e.input.length)+" or more characters"}
it isn't possible to translate the text in the JS file like that.
And please notice that, supporting customization code is beyond our scope of support. If you want to customize your site, we offer a customization service. You can contact us here for more details https://metabox.io/contact/
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