Okay, but it would be nice to be more flexible to have changes via LocalFile Editor for the Admin theme as well.

For me, the window is just to small to see it all, even when scrolling - but of course this is just personal.

Pity this issue (for me) is still in version 14.2.0, just a recap for myself:
For 14.2.0 modify admin/themes/default/theme.css line 6227, change 45px to auto !important:
  6226  #action_add_tags .selectize-input.items.not-full.has-options.focus.input-active.dropdown-active, .selectize-input.items.not-full, .selectize-input.items.full {
  6227          padding: 10px 10px; height: auto !important; box-shadow: none; border-radius: 0px; border: 1px solid #787777a6;
  6228  }

I know the LocalFiles Editor can't change the CSS for the administration. But here is a trick. Use [extension by ddtddt] Add < head > Element instead with:

<style>
#action_add_tags .selectize-input.items.not-full.has-options.focus.input-active.dropdown-active, .selectize-input.items.not-full, .selectize-input.items.full {
  height: auto !important;
}
</style>

and you're good. No need to re-implement it after each Piwigo update.

A slight improvement could be to show a 2 lines input by default, auto adjust height depending on content, until you reach 6 lines. Then you have to scroll in the input field.

  height: auto !important;
  min-height: 76px;
  max-height: 184px;