I always work with two tabs of code, simply because a class is rarely interacting with itself so you want both sides of the pipe :).
Without Eclipse, It's a linux, terminator with 4 terminals in there and a two-paneled sublime text. I don't need my second screen for that, alt tab does the job. :)
I tried to work on Eclipse with a 9' laptop in an airport once, thought I would die before the plane take off :).
At work i use a laptop with a 24 inch display. I have the laptop to display Eclipse debug and other info that i might need and have Eclipse mainly on my 24inch.
At home, doing web dev i have a 27 and a 22. It is a lot more usefull there as i have my eclipse on the larger screen and have the browser and shell on the 22 inch. No need to switch between windows and tabs.
It safes a lot of time and increases performance drastically.
As far as physically managing two screens: I have a laptop dock at my desk which has a display port. Drop my laptop in and instant additional screen. Come home and sit on my couch, plug in HDMI and use my TV as a monitor, really great for web development.
I use a 50" 4k monitor and a 42" 1080p monitor.
The time it saves easily covers the cost.
I wouldn't use less then 2 x 30" monitors.