I use my TV as a monitor sometimes, and it should be around the size of those monitors, maybe bigger.
Its not the size of the monitors together that matters, otherwise I'd just run my computer off a 100" TV screen. Its about the resolution size.
Example: A regular 1080p TV has a resolution of 1920 x 1080.
My current set up runs at a 5400 x 1920 resolution.
Short Version: I have more screen space available to be used.