EGOL previous kick at this can: What Width Are You Designing For Now ?, February 2013.
At which time I commented:
My page templates are and have been for years 1200-1500px page max-width. And adjust via media queries (responsive design) to accommodate smaller screen sizes.
Note: there is also some (still testing) adjustment to accommodate device resolution differences within similar screen sizes.
I don't worry about 'huge' desktop monitor size as (1) not all maximise browser windows, (2) many browser viewports are filled with top and sidebars, and (3) I don't want the text width to become a poor reading experience by being too wide.
To expand a bit about why I exceed your 1000px limit...
* I build from content width.
* that width is calculated by:
---site font size: I use 16px = 1em.
---the optimal reading width derived from testing: 50 to 60em.
This means that at the low end content width is 800px (50*16), at the top end 960px (60*16)
Then I add one or more sidebars/columns whose widths vary by their content requirements, plus margins/padding.
Note, also that 1200-1500px is the max-width allowed. If the desktop viewport is constrained it sizes down. Plus of course once media queries are invoked things change quite a bit, gotta love the power of CSS.
I have actually come down in max size as I've since done an entire layout redesign across all devices in my change to contextual delivery. It's now 1000-1200px max-width on desktop as I switched since then to a main with a right sidebar layout. However, I'm currently playing with several 'future proof' (ha!) designs that play off the viewport (see further below).
First back to some basics:
Few webdevs use points as a measure, most use PX's (pixels) in whole or in part because while it is considered a relative unit (contrasted with points, inch, mm, cm et al) it is also absolute in terms of a given screen resolution; it can vary greatly between screens but remains consistent for each.
EM's (in CSS) are derived from user defined DPI, the particular font specified, and the current element font-size. This means that it varies based on the size of the parent; each element may be different. REM's (root em's) are less variable as each varies as specified only from the root, not it's parent.
There can be a problem with setting the document HTML font-size in px's - it can, depending on browser, over ride user settings and/or zoom to the detriment of accessibility. My long standing practice is to set the base font-size via percentage. The standard OS/browser default is 16px (medium). Thus setting font-size: 100% is - usually - setting it to 16px. Regardless, it sets it to the system default from which the user may customise.
Note: there are a lot of CSS length units.
The <length> CSS data type, web docs, Mozilla Developer Network.
How many were new to you?
I asked because... just to make things more
complex fun I should mention vh and vw CSS3 units. Why? Because they are quite likely the way forward.
* vw: viewport width
* vh: viewport height
* vmin: the smaller value of the viewport’s width and height
* vmax: the larger value of the viewport’s width and height
Note: IE 11 and Edge do not support vmax.
Note: if you have a large user base on older browsers aka broadly before this year's version(s) this is strictly a progressive add-on to existing.
And you should also get comfortable with the CSS3 calc() function. Ha!
Rather than my rambling on I recommend reading Responsive Font Size And Fluid Typography With vh And vw Units by Michael Riethmuller, Smashing Magazine, 10-May-2016.