The concerns are:
- the sled needs to keep tension on both chains and if it gets too nearly beneath one motor, the other chain doesn’t have enough tension for accuracy. Space the motors wider if growing the work area wider.
- Great force is needed to pull the sled up near the top of the sheet where the chains become nearly horizontal. Move the motors higher to keep them well above the top of the work area to avoid the wide angle.
You can work on this in a CAD program (good practice for learning - pick one you’ll use for designing things to cut once the machine is built and running 
) or with pencil and paper.
A good starting point might be the well-documented design in this thread. @dlang has put a lot of thought into the forces involved, and provided the software for exploring the geometry that you mentioned.
It looks to me like the beam across the top could be 12 feet long, and the verticals extended by a foot, and you would keep chain angles similar to the stock setup over a 5’x5’ panel. Getting to 8’x4’ portrait orientation while keeping the angle would probably grow beyond the 10’ height you mentioned.
Note that you’ll probably need longer chains for these modifications. Bar and Hannah don’t offer chain in their store - yet, nudge, nudge - but you’ve got time to calculate how much you’ll need to add and get some.