The sectional will define the room and does not need to be centered on the window. 3 feet should be enough room for the buffet. Lay out the sofa in painter's tape for size and position.

An L sectional with a big ottoman for the feet. A comfy carpet and pad and big sturdy throw pillows make good floor lounging.

May we have a hand drawn sketch of the room with measurements of each wall, openings, windows, the fireplace, etc. then we can draw up a 3D rendition so we can answer your question regarding a larger couch. Why a sectional? They often take over the room and control what you can or cannot do in the room. This room looks like it is too small for a sectional. We can help more once we have some measurements. Best Wishes.