This is a difficult question for some reasons.

1. One screen doesn't usually cut it.
2. Most screens lack resolution real estate.
3. A laptop should be used to do on premises coding when necessary, it's not nearly what most coders would like as a main tool.

Having multiple screens does make the life of your average coder easier. Whether it is having both source code and result in one view or adding a third screen for reviewing sources a single screen on a laptop just doesn't cut it. We make it work.

More important then screen size is screen resolution, I'd rather have a 13 inch Full-HD screen then a 17 inch 1600*900 one.

A laptop is a tool, as is a desktop or a power drill for that matter. If I need to fix something on premises any laptop will do, the resolution would determine how efficiënt I can be not the screen size.

Hence the old saying, it's not the size of the tool it's how you use it.