Hello ,
Change in the monitor size will not result in sensitivity change however resolution change does .
Changing resolution in 2D games will result in change in the sensitivity and the same goes for changing resolution and forcing the game to run a different aspect ratio stretched .
if you are changing resolution make sure to do it in the same aspect ratio if possible .
Also if you are using a new mousepad i really suggest you break-in the mousepad first and changing the sensitivity later , most pads change their coating after the first wash and or after some time of using .
these two sheets are really helpful for mousepad speeds :
https://docs.google.com/spreadsheets/d/1RAnmZxDNduaGV8kB-GCvZ0MO6d9-0j9jmrU2f8dp0Ww/edit#gid=0
https://docs.google.com/spreadsheets/d/1jEXzsCHZVXN9c-wJsnTOBIsbfgzWBfKz7iYXbHIfet8/edit#gid=52719204
Personally i would say yes you should change your sensitivity After giving the mouse some time and getting used to it , if your new mouse allows you to have a finer movements there is no reason to not take advantage of that .
also depending on how old your last mouse was you should check to see if the mouse has/had prediction and built in acceleration or not .
i believe GuardiaN was using a mouse that was like that actually but i'm not sure if he is still using it or not .