What's a better way of finding all of the parts/unionoperations in a certain area?

After learning how to use Region3 efficiently, I realized it is greatly limited. It can only return 20 parts at max! Is there a more efficient way to find ALL the parts in a region? I'm not looking for someone to "fix" my script, I'm looking for a method or a solution to my problem. Thanks!

My code utilizing Region3

function drawRegion(Start,End)
	return Region3.new(
		Vector3.new(
			math.min(Start.X,End.X),
			math.min(Start.Y,End.Y),
			math.min(Start.Z,End.Z)
		),
		Vector3.new(
			math.max(Start.X,End.X),
			math.max(Start.Y,End.Y),
			math.max(Start.Z,End.Z)
		)
	)	
end