I would recommend to use this addon:
https://www.curseforge.com/wow/addon...dly-nameplates
or at least the code.
You can adjust the size as you want and theres space to set up other options.
Friendly nameplate addons do not work in dungeons but arenas.
There are limitations but the size will stay as set even in dungeons.
Most overall limitations are related to the anchors and timers, so addons cant mess around with setting up special timers/frames above nameplates while in combat.
The size will only resett by making changes to the cvar within the instance and for this example by pressing the cvar macro over and over.