discourse/app/assets/stylesheets/common/components/file-size-input.scss

.file-size-picker {
  display: flex;

  .file-size-input {
    flex: 1;
  }

  .file-size-unit-selector {
    flex: 0 0 8em;
  }

  .select-kit .select-kit-header {
    border-left: 0;
  }
}