You can set the DH Key Size adding the following line in server.properties.

jdk.tls.ephemeralDHKeySize=2048

You can setup custom cipher lists by adding comma separated ciphers like:

ssl.cipher.list=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, ....

Check http://www.xeams.com/server-properties.htm for details about server.properties file.