As it so often happens, I went out for a walk and came back to look at a few other things.  And sure enough, I now see this would be how you tune the job.  I guess if any good can come from my lack of attention to detail, at least I now have it engraved in my mind.

sqoop import -D mapreduce.map.memory.mb=4096 -D mapreduce.map.java.opts=-Xmx3000m ....