The number of dots per inch that your computer graphics card can generate needs to be matched to the number of dots per inch your external monitor can display. More dots per inch (dpi) equals more fine detail or more information being displayed on the screen, e.g. you can physically read more cells of a speadsheet at once.
Strange though it may seem, it is possible to have too big a monitor. If the pixel size is very small, you may need to have the minitor mounted closer so you can see the fine detail. That then leads to constant head movement to take in all the screen. Then there is the varying distance between your eyes and the edges and centre of the screen. Your eyes will be constantly refocusing as you scan across the screen causing tiredness and headaches.
A 17" laptop display is pretty much as large as you want to go at the distances laptops are viewed at. If you want more screen real estate for video editing or similar, consider a second monitor mounted side by side with your existing laptop screen. Programs can be slid sideways from one screen to the other, or one screen can show the work in progress while the other contains editing tools and the like. Ideally the two monitor displays (laptop and external) would have the same dpi settings.
My son used this sort of set up on his university studies. One screen contained his coursework, the other set to display music, games, videos and social media. He did get a couple of degrees, so it worked for him, although I am at a loss how he ever managed to concentrate with all that stuff going on!
The two monitors don't have to be the same size or necessarily the same resolution, but it does make it a bit harder switching your attention between the two if they are not.
Monitors can be mounted one above the other rather than side by side if that suits your purposes better.
Finally, I note your age as 47. Do get your eyes checked by an optician and consider wearing corrective lenses just for computer work if necessary. That is the age that many people start to struggle switching between near and far focus. Just physiological changes in the eye because of age. Computer screens can be very demanding of your vision when you work with them for long periods. As I mentioned above, there is a constant micro refocussing going on as you scan across the screens, unless you have wraparound type screens. That is the source of many people's computer headaches and migranes, even though they are not consciously aware of any problem, because there isn't one technically speaking.
Last edited by G0ULI; 14th February 2017 at 22:59.