Heated Bed vs Heated enclosure for large scale printer
I tend to agree with Davo, that you might want both. But I'd probably try the enclosure first. My printer is about 1200 by 400. I looked at options, and found some difficult trade-offs:

First, the big heatbed approach: 


as you know, getting a single heatbed that big will be expensive (and if it ever breaks or fails, you have to replace it all).
a big heatbed also draws far more power than a RAMPS board can switch, so you'll have to use the on-board control to control a power relay (solid state or mechanical). 
a big heatbed will also waste a great deal of power when you're doing small things.


Second, the heated enclosure approach:


more costly to heat up at the beginning, but if insulated it may be cheaper for long prints (with a bed that large, your prints might take really long, too) because it can retain the heat better.
if you keep the air circulating inside, you'll get much more uniform heat, rather than hot first layers and cooler higher layers.
consider the effect of the heat on all the other components: motors, electronics, pre-heating the fibers, thinning any lubricants,....
you could save some heating cost and time by providing a way to shrink the space to be heated: perhaps just a partition you can insert when doing smaller prints. It wouldn't have to be nearly airtight to make a big difference.


A third option is several small heatbeds:


this lets you turn on just the ones you need for any given print
still expensive, but cheaper than one big one, especially when any of them fails.
there will be uneven heat at the seams, but if you place the boards tight together I doubt it's enough to matter. You could also carefully trim the boards' edges to get the spacing closer to uniform.
temperature regulation will be tricky. If you want to support a temperature sensor for each board, you'll have to start hacking at the control software, because (as far as I can tell) there's no provision for multiple heaters. Probably easier to create a completely separate unit with a big power supply, and a simple thermostat for each of the beds, that you just set manually before starting a print.


Overall, I think the heated enclosure may be best. I like the hair dryer idea or heat gun idea, of course you'll need a little extra circuitry to switch it, but you can use the usual software for temperature regulation by installing thermistor or thermocouple in the case (just be careful about circulating the air well enough to avoid "hot spots"). 

You may want to add an extra cutoff to protect against runaway heating -- I had that happen once when a thermistor literally fell out of the mounting hole in the hotend. Something like https://www.grainger.com/product/SUPCO-Thermostat-407L11.

Let us know how it turns out.