Doesn't scale on HiDPI on Linux. Increasing font size only makes a part of the app readable and there is no zoom in/zoom out functionality that I could find. :(
We pick up the default GTK dpi settings, and if that's not suitable, you can use the dpi_scale setting to set it specifically (you'll have to restart Sublime Merge after changing it though).
I run Sublime Merge on high dpi screen under Linux every day FWIW.