Is there a way to set this value within a SQL Worksheet
No, not currently. The preference Sql Array Fetch Size applies to all connections. It looks like a developer once thought of changing this preference value whenever SET ARRAYSIZE runs in a worksheet, but ultimately decided not too -- what's needed is a fetch size property scoped at the connection level or even the worksheet level. Since a worksheet can use an unshared connection, that would provide users with precise control.
You might want to add this as a Feature Request on the SQL Developer Exchange: http://sqldeveloper.oracle.com/
Of course, it should not be necessary to reset a connection to pick up this preference change, but may in fact only work that way in some cases. I could be wrong, but probably the result grids will pick up the new value without a connection reset.
SQL Developer Team
Thanks, in my testing it did require a reset of the connection.