I'm seeing some inconsistent X resolution on my system76 oryx laptop,
and am seeking some insight.

Things seem OK as I boot KDE Neon and log in as user_a. When I then "New
Session" from User Switcher widget, I notice that the X resolution has
changed: at the user prompt in the display manager, the highlighted
user's (.face) image is now 21mm wide (was 30mm before), and after
logging in as user_b, the expanded Application Launcher window is now
112mm high (was 173mm - nearly the whole height of the display). Ditto
for any additional spawned sessions, but if I head back to user_a via in
User Switcher, I see the larger X resolution at the password prompt.

It doesn't seem to be tied to any specific user: if leave user_b's
session running, switch back to user_a and log out, get back into the
user_b session, and then "new session" into a fresh user_a, then the
smaller X resolution occurs. And, all the same behavior is noticed if I
start with user_b instead of user_a.

I'm using version 435 of the nvidia driver via the system76-driver-
nvidia package. I do that because when I use the nouveau driver,
the display will not wake up from a suspend.

xrandr -q reports the following for the larger X resolution:

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    Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 16384 x 16384
    HDMI-0 disconnected (normal left inverted right x axis y axis)
    DP-0 connected primary 1920x1080+0+0 (normal left inverted right
            x axis y axis) 344mm x 194mm
       1920x1080     60.01*+
    DP-1 disconnected (normal left inverted right x axis y axis)


xrandr -q reports the following for the smaller X resolution:

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    Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 65535 x 65535
    HDMI-1 disconnected (normal left inverted right x axis y axis)
    eDP-1 connected primary 1920x1080+0+0 (normal left inverted right
            x axis y axis) 344mm x 194mm
       1920x1080     60.01*+
    DP-1 disconnected (normal left inverted right x axis y axis)


KInfocenter | Graphical Information | X-Server reports the following for
the larger X resolution:

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    Dimensions   1,920 x 1,080 Pixels (344 x 194 mm)
    Resolution   143 x 144 dpi


vs the following for the smaller X resolution:

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    Dimensions   1,920 x 1,080 Pixels (508 x 285 mm)
    Resolution   96 x 96 dpi


(Notice that for the smaller X resolution kinfocenter reports 508x285mm
while xrandr reports 344x194mm.)

I find the smaller X resolution more attractive, and would prefer that
for all sessions, but I'd prefer consistency above smallness. Any
thoughts on how to get consistent behavior?

There are several Neon versions out there, please be more specific about your exact Plasma version when asking a question.

$ grep PRETTY /etc/os-release && plasmashell --version
PRETTY_NAME="KDE neon User Edition 5.17"
plasmashell 5.17.5

More info...

• I've been able to duplicate the problem with the solus 4.1 distro (also running plasma 5.17.5). So, it's not just neon/ubuntu/debian, since solus is independent from those.
• when I uninstall the nvidia driver, the desired smaller resolution is always used (after a reboot, of course): under the initial login session, "xrandr -q" is consistent with the smaller resolution numbers I cited earlier (eDP-1, min 320x200, etc.)

Sounds not so much like a Plasma problem but a Nvidia one, proprietary drivers are not something we have any influence over I am afraid.