https://www.reddit.com/r/Crostini/wiki/howto/adjust-display-scaling
The BEST way to do this in ChromeOS is to right click the application icon and choose "Use low density", then either allow ChromeOS to restart the app or say "Not now" and do it yourself. Upon starting the application again you should have a readable UI without monkeying with the Java VM options or the properties of whichever app your are using.
Default launch path and arguments from the Flatpak of PyCharm-Community installed in LinuxVM Terminal These result in a very tiny and hard to read UI.
/app/pycharm/jre64//bin/java \
-classpath /app/pycharm/lib/bootstrap.jar:/app/pycharm/lib/extensions.jar:/app/pycharm/lib/util.jar:/app/pycharm/lib/jdom.jar:/app/pycharm/lib/log4j.jar:/app/pycharm/lib/trove4j.jar:/app/pycharm/lib/jna.jar \
-Xms128m \
-Xmx750m \
-XX:ReservedCodeCacheSize=240m \
-XX:+UseConcMarkSweepGC \
-XX:SoftRefLRUPolicyMSPerMB=50 \
-ea -Dsun.io.useCanonCaches=false \
-Djava.net.preferIPv4Stack=true \
-Djdk.http.auth.tunneling.disabledSchemes="" \
-XX:+HeapDumpOnOutOfMemoryError \
-XX:-OmitStackTraceInFastThrow \
-Dawt.useSystemAAFontSettings=lcd \
-Dsun.java2d.renderer=sun.java2d.marlin.MarlinRenderingEngine \
-XX:ErrorFile=$HOME/java_error_in_PYCHARM_%p.log \
-XX:HeapDumpPath=$HOME/java_error_in_PYCHARM.hprof \
-Didea.paths.selector=PyCharmCE2018.3 \
-Djb.vmOptionsFile=/app/pycharm/bin/pycharm64.vmoptions \
-Didea.platform.prefix=PyCharmCore \
com.intellij.idea.Main
Adding the below options don't seem to affect the UI scaling in the LinuxVM on ChromeOS on a Pixelbook at "Looks like 1714 x 1143". Some of the notes I've found mention Java 9 might fix this, or having the "scaling factor" set to an even multiple of 100% ie 200% or 300%. Sadly the ChromeOS settings doesn't really tell you what the scaling percentage is.
-Dide.ui.scale=2.0
-Dsun.java2d.uiScale.enabled=true
The sort of workaround is to increase the font sizes and save a profile using the HiDPI Profiles plugin if you frequently change between a HiDPI resolution and lower resolution monitors.
https://youtrack.jetbrains.com/issue/IDEA-171404#focus=streamItem-27-2102918-0-0