How can I get my laptop's monitor size?

Another option, using xrandr, is the command:

xrandr | grep ' connected'

Output:

DVI-I-1 connected primary 1680x1050+0+0 (normal left inverted right x axis y axis) 473mm x 296mm
VGA-1 connected 1280x1024+1680+0 (normal left inverted right x axis y axis) 376mm x 301mm

(mind the space before connected, else disconnected will be included)

Important differences between xdpyinfo and xrandr

• While xrandr lists screens separately (in case of multiple monitors), xdpyinfo outputs one single set of dimensions for all screens together ("desktop size" instead of screen size)

• As noticed by @agold there is (quite) a difference between the two, which seems too big to be a simple rounding difference:

  xrandr: 473mm x 296mm
  xdpyinfo: 445x278
  

It seems related to a bug in xdpyinfo. See also here.

If you'd insist on inches

Use the small script below; it outputs the size of your screen(s) in inches; width / height / diagonal (inches)

#!/usr/bin/env python3
import subprocess
# change the round factor if you like
r = 1

screens = [l.split()[-3:] for l in subprocess.check_output(
    ["xrandr"]).decode("utf-8").strip().splitlines() if " connected" in l]

for s in screens:
    w = float(s[0].replace("mm", "")); h = float(s[2].replace("mm", "")); d = ((w**2)+(h**2))**(0.5)
    print([round(n/25.4, r) for n in [w, h, d]])

To use it:

Copy the script into an empty file, save it as get_dimensions.py, run it by the command:

python3 /path/to/get_dimensions.py

Output on my two screens:

width - height - diagonal (inches)

[18.6, 11.7, 22.0]
[14.8, 11.9, 19.0]

Edit

Fancy version of the same script (with a few improvements and a nicer output), looking like:

Screen  width   height  diagonal
--------------------------------
DVI-I-1 18.6    11.7    22.0
VGA-1   14.8    11.9    19.0

The script:

#!/usr/bin/env python3
import subprocess
# change the round factor if you like
r = 1

screens = [l.split() for l in subprocess.check_output(
    ["xrandr"]).decode("utf-8").strip().splitlines() if " connected" in l]

scr_data = []
for s in screens:
    try:
        scr_data.append((
            s[0],
            float(s[-3].replace("mm", "")),
            float(s[-1].replace("mm", ""))
            ))
    except ValueError:
        pass

print(("\t").join(["Screen", "width", "height", "diagonal\n"+32*"-"]))
for s in scr_data:
    scr = s[0]; w = s[1]/25.4; h = s[2]/25.4; d = ((w**2)+(h**2))**(0.5)
    print(("\t").join([scr]+[str(round(n, 1)) for n in [w, h, d]]))

EDIT 2 (May 2019)

"Kind of" on request (in a comment), a modernized / more advanced / improved (no system calls, no parsing but using Gdk.Display) version, doing pretty much exactly the same:

#!/usr/bin/env python3
import gi
gi.require_version('Gdk', '3.0')
from gi.repository import Gdk

dsp = Gdk.Display.get_default()
n_mons = dsp.get_n_monitors()

print(("\t").join(["Screen", "width", "height", "diagonal\n"+32*"-"]))

for i in range(n_mons):
    mon = dsp.get_monitor(i)
    mon_name = mon.get_model()
    w = mon.get_width_mm()/25.4
    h = mon.get_height_mm()/25.4
    d = ((w**2)+(h**2))**(0.5)
    print(("\t").join([mon_name]+[str(round(n, 1)) for n in [w, h, d]]))

Output:

Screen  width   height  diagonal
--------------------------------
eDP-1   20.0    11.3    23.0
HDMI-1  18.6    11.7    22.0

I will leave the original answer, since it seems inappropriate to remove the answer after such a long time, that generated the existing votes.

Just in case you want a more general answer, you can cut the the gordian knot, and use a non-geeky physical ruler for that. As per this wiki, the "size of a screen is usually described by the length of its diagonal":

If you have a ruler that only displays centimeters, you can use the simple conversion:

1 cm = 0.393701 in
(or 2.54 cm = 1 in)

So if your ruler measures 30 centimeters, your screen is 11.811 inches. You can also use google with a query of the form 30 cm to in.

Image credits: https://en.wikipedia.org/wiki/File:Display_size_measurements.png

How'bout this :

xrandr | awk '/ connected/{print sqrt( ($(NF-2)/10)^2 + ($NF/10)^2 )/2.54" inches"}'