Small font size with 1440p external monitor
In my experience if you connect your monitor via HDMI it won't give the the font scaling options in System Preferences > Displays.

If however you connect via DisplayPort i.e. Mini DisplayPort to DisplayPort it should give you the options similar to these:



There used to be some hacks to trick the monitor into switching into HiDPI mode but I couldn't personally get these to work on macOS High Sierra, but using a DisplayPort cable rather than either HDMI or Mini DisplayPort to HDMI did resolve the issue.