Overall, I think you'll be happier with a sofa and two chairs. If you're set on a sectional, try a loft/apartment sized flex or bi-sectional. They are easier to arrange differently because the chaise portion can be moved from left to right when you decide to move the furniture. You will regret filling the room with a monster sectional, because you'll be stuck in a rut with no possibility of changing things up when you tire of the arrangement.

Agreed on the single sofa/smaller sectional and two chairs point. With the stairs and railings and chandelier this room has a look that makes me think more of a formal living room than a family room. It could definitely serve double-duty, but I think it needs pieces that are a little more upright.

I have two young kids and a pottery barn slip covered sectional (have had it for 4 years now) and LOVE it. It works for our family room and tv viewing. I don't have one quite as large as yours pictured, but love that I can sit in the corner and relax or sleep. I think minus one segment from the couch you pictured, your room could handle a couch that size. You may not have easy access to get in and out...but I think comfort and having something the kids and adults can feel comfortable lounging in (and washing!) is important. Think cozy and contained : ) I also prefer the actual sectional (not chaise) since the chaise doesn't really offer comfortable additional seating for guests (a back)--not as functional--and I think the chaise couches look more informal than traditional sectionals... the lazy man's sectional so-to-speak. Just my opinion.