On the left first floor - are those double doors or double windows? Does the "proposed screened to be area" come off the kitchen? If not what room? What will be the purpose of the screened are (eating or sitting?) and what is the purpose of the new deck area (eating or sitting)? If it is double windows on the left - what room is that? - and could you make those into double doors with access onto your deck?

I can't address shape so much as I can say make it big enough that it serves the purpose you have in mind and 2 entrances and exits would make it so useful! How would a deck extension affect the light through the windows on the lowest level on the left?

And just to play devil's advocate - have you considered putting the screen room at the left end of the new deck? Again - depending on your purposes and if you have direct access from the house.

Currently they are windows on the left first floor, but those will come out be french doors leading from the dining room to the exposed deck area. The current slider will also be changed to french doors leading from an area between the kitchen and living area (Great Room style) to the screened in porch. The screened in section will be mainly used for eating and also a small couch for reading and the exposed section will also have a table for eating and a couple adirondack chairs for hanging...