From the docs the syntax of the storage clause is:

STORAGE
({ INITIAL size_clause
| NEXT size_clause
| MINEXTENTS integer
| MAXEXTENTS { integer | UNLIMITED }
| PCTINCREASE integer
| FREELISTS integer
| FREELIST GROUPS integer
| OPTIMAL [ size_clause
| NULL
]
| BUFFER_POOL { KEEP | RECYCLE | DEFAULT }
}
[ INITIAL size_clause
| NEXT size_clause
| MINEXTENTS integer
| MAXEXTENTS { integer | UNLIMITED }
| PCTINCREASE integer
| FREELISTS integer
| FREELIST GROUPS integer
| OPTIMAL [ size_clause
| NULL
]
| BUFFER_POOL { KEEP | RECYCLE | DEFAULT }
]...
)

I'm not sure exactly what you meant by your statement "room for rows to double in size when they are update", but I assume you at least want something like:

STORAGE (INITIAL 10240K NEXT 10240K PCTINCREASE 0)