Save the file in it's default state (size etc) then go to:

(File / Render toDisk... / in the sub screen, type in the size and resolution you want )

Sub menu comes up to name the file/format/and folder where you want it.

I always rename the new file to define its larger size.

I do not have Version 7, only 6, though I'm sure it;s the same.

p.s.  also remember that any mask or dof renders will need the same procedure.

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Ockham's razor- It's that simple