This is related to this issue: https://www.drupal.org/node/3013611 → . which solved the size for the second displayed select box.
This patch should fix the small size of the first displayed select box, where you choose the terms to be merged.
I used CSS view height 60% to make the size responsive to whatever size screen people were working off. Feel this is more adaptive than a fixed number of rows.
I also gave the select box a class, so that people could add their own styling via css.
Needs work
1.0
Code
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Issue ✨ Use checkbox and radio instead of Select, Provide better user experience Needs review makes the selection of terms much easier by putting them in a select list with radio buttons. Can you review that fix and mark it as "Reviewed and tested by the community" if it works for you? (And close this one if your needs are met?)