I have few question regarding waves node

I setup a new waves node on Ubuntu machine , i am going to use this node only for multiple users wallet.i have these following questions.

1.i want to disable miner feature in conf so this will not effect wallet operations ?
2. i read in forum to control java memory usage but still i am unable to to use -Xmx parameter , how to start node with this parameter. my server have 4 GB memory and now node consuming 75% , i want to limit node to not use more then 2 GB of memory.
3.Is there any way to only keep few latest blocks data ?