4096 MB should be sufficient unless you want to launch Cloudera Manager. All of the CDH services should be able to start up and simple examples should work pretty reliably.
As for your issue, I would suggest going to File -> Import Appliance... and opening the .ovf file in the directory you unzipped. That file will set up the virtual hardware for you (instead of you setting up everything but the hard disk) so it eliminates a lot of variables. Not sure what the root cause of the issue is, but that's a good first step.