Hi - I also have this problem, On a Dell Precision 5510. My laptop has a 4K screen, but when working at my desk I have the laptop lid closed and instead display on two 1920x1080 monitors. I think the problem started sometime after I was working away from
my desk, and adjusted scaling for my laptop screen because 4K is just too small. After returning to my dual-monitor desk, I noticed the mouse cursor was small in Word/Outlook. :-(
So, if a solution is found privately please report it to this forum. Thanks! :-)
Here's some of my own screenshots. Here in Control Panel you can see my Mouse pointer has converted to the finger and is normal size:

But here in a Word document - when I mouse over a link... it's itsy-bitsy small!

Yet in Excel it seems the cursor size is fine:

And in Powerpoint?:

yeah - that's fine too. :)
...now, I can tell you outlook has this problem too - when reading an email, any time I mouse over a link, the Pointer becomes a itsy bitsy finger. It makes sense to me that outlook is affected, because by default, Outlook uses Word for reading/editing
emails.
And similar to zzzzt's posts: When I mouse in word, either between pages (where the double-clicking is supposed to hide whitespace or into the left margin where the pointer becomes a reverse-arrow and clicking should highlight a line, the cursor shrinks
in each case.
So this is definitely tied to Word, and because of that; Outlook. But other office apps don't appear affected, and non-office apps also work fine.
I hope you can figure this one out for us.