On Linux, setting some environment variables for Qt worked for me:
Lower scale factors = smaller. I set mine to 1.2 on a Thinkpad T450s (1920x1080)

Source: viewtopic.php?t=34104#p73892

It refers to the next LMMS version, 1.3, which is the "Nightly" version right now.

While running LMMS 1.2.2 on Debian Buster with XFCE4, the fonts are huge! Everything else on the machine is reasonably sized, and in 1.1.x LMMS had the tiny fonts that so many have asked about. I'm thinking I might have a stray setting or two if I tried to enlarge fonts in the past, but I can't find...