The new Release 20.03 is out! You can download binaries for Windows and many major Linux distros here .
In recent nighlies it's : Settings / Environment / View / Toolbar Icons Size.But on 17.12 version, I don't remenber if it was implemented.You can try the recent version provided by Xaviou in : https://wxstuff.xaviou.fr/article/debian-repository.htmlgd_on