These funnels have a screw on attachment.
when using this funnel/base, is part of the outer perimeter of the filter covered by the funnel? or Is the entire surface area of the filter usable?
Part of the filter is sandwiched between the funnel and base. This ensures that the edges of the filter are sealed and none of the liquid to be filtered escapes around the filter.
No, they are only available in 300 mL sizes.