Your .ini file should look mostly like this:
algebraic:~ loki$ more /Applications/KNIME\ 4.1.0.app/Contents/Eclipse/Knime.ini
-startup
../Eclipse/plugins/org.eclipse.equinox.launcher_1.4.0.v20161219-1356.jar
--launcher.library
../Eclipse/plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.551.v20171108-1834
-vm
../Eclipse/plugins/org.knime.binary.jre.macosx.x86_64_1.8.0.202-b08/jre/Home/lib/jli/libjli.dylib
-vmargs
-server
-Dsun.java2d.d3d=false
-Dosgi.classloader.lock=classname
-XX:+UnlockDiagnosticVMOptions
-XX:+UnsyncloadClass
-XX:+UseG1GC
-Dsun.net.client.defaultReadTimeout=0
-XX:CompileCommand=exclude,javax/swing/text/GlyphView,getBreakSpot
-Xmx2048m
-XstartOnFirstThread
-Xdock:icon=../Resources/Knime.icns
-Dorg.eclipse.swt.internal.carbon.smallFonts
algebraic:~ loki$
The line you’re interested in is the one starting -Xmx - in the above example it is allocating 2048MB. You can use either m or g in your specification (to denote MB or GB.)