Just for a rough guide: -
you will need at least 9Gb just for your oracle /dboracle s/w and how big is your dbase, xGb? Will you want oracle archiving?
Root=4Gb, swap=4Gb, var=2Gb
So 10Gb for Solaris, 10Gb for oracle s/w, 5Gb area for logs?10gb area for you database.
Adjust accordingly, maybe increase swap a bit