You have to know how much juice the thing draws..... Get a "Kill-A-Watt" meter and plug your plotter into it. Then you can see what it draws so you can match it up with the right sized battery using the APC link above
Besides... in the event of a power outage you are going to have lots of other devices to protect than the plotter.
Lights, Computer, Monitor, Ethernet Switch, Router, Cable Modem, printer(s), Cordless Phones, etc...
For me, I use 4-5 of the 750's that SARex posted... one for each PC (Design, RIP, Accounting, Sublimation, LaserEngraver), and one for the Mutoh.
The Mutoh has 3 plugs, two for heaters and one for the printer. The heaters pop the breakers on the 750VA UPS so I just protect the printer itself. The heaters get regular old city electricity.
My disaster scenario is to protect things long enough to finish whatever job is running, save my open files, and get things shut down. To stay up & running longer than 10minutes we'd have to add a generator and a transfer switch. Many K's of money!