Right now the only way to specify the spark-shell memory is to use the now-undocumented SPARK_MEM environment variable. I have raised this issue on the JIRA ticket that deprecated SPARK_MEM (https://spark-project.atlassian.net/browse/SPARK-929), and the resolution there may affect how we want to proceed here.

(As an intermediate solution, you can always just add an option which sets SPARK_MEM, and we can change that later to something like SPARK_DRIVER_MEM when we add it.)