In OpenShift CLI, how can I define the memory size within the oc new-app command?
I do not think that oc new-app allows you to specify this on the command line. So your best bet would be to use oc new-app and then use oc patch to edit the created Deployment.
Alternatively, you can use oc run to start a single Pod, with that command you can actually specify --limits and --requests, but that is probably not what you want:
oc run ruby-hello-world --image=ruby-hello-world --limits=cpu=200m,memory=400Mi --requests=cpu=100m,memory=200Mi