For this kind of specific dimensions, I would go the DIY route... Such big printers does exist, but are expensive.
You can go for a modified solidoodle with extended Y carriage, and find a bed that will be big enough. You could make an aluminium surfaced plate, and stick 4 bed heaters underneeth it, controled by 4 SSR on the same card output. But it will be a huge work. You'll have to go for a bigger rods dimension, 12 mm or 16 mm to carry that weight, and your machine will be slow.
Now with all that, you'll have to design and make work a new Z axis system, for that monster plate, not talking about the numerous powersupply you'll need to heat 800 mm of bed...
All in all, it might be worse considering slicing your prints into small ones that you can glue together...
SD3, early 2013. Silicon 200W 8x8 heater bed, lawsy carriage on 8 mm hardened chrome shafts, E3D V6 and lawsy MK5 extruder, ATX 350 W PSU, Custom build plywood enclosure, azteeg x5 mini smothie firmware, with vikki 2 screen, fishing line conversion and pulley conversion, M5 Z rod. Calling it Gran'ma, going strong ! <3