Forerunner 955 - Is it possible to change the font size?

No. Sadly not. I have the opposite problem. Especially on Epix.

+1 for making it adjustable. The funny thing is that the 945 showed more content even with its smaller sized screen! Here I attached a photo demonstrating the difference. If you look at the 955 screen, there is a big surface not used by the notification text. It is wasting precious space. 945 uses the available space so much more efficient. 
Garmin, please, adjust the layout a bit to fit more content! (Btw, the white background with black letters is so much more readable, especially in darker lighting conditions!)

As for the black screen I thought it got better, now how much notifications could really adjust, scrolling the notifications doesn't make sense. You know before version 13.22 was showing up all the information from twitter and then it was terrible.

Didn't this new beta fix it? Change Log

Improvements to Altimeter values
Improvements to graphs including scrolling via touch
Improvements to alarms and timers
Improved organization of glances
Various fixes in activities
Various fixes for watch faces

Glances are widgets, it's probably about organizing them in folders (fenix/epix feature).

Yes, 955 screen are is not used in best way. Small fonts and a lot of free wasted space

This is fixed with 15.11 please confirm 

Can you please compare now with 15.11. i think much more content is visible now

it is indeed fixed in 15.11

more text is visible

Unfortunately they made fonts even smaller.

Are they thinking only people of 30yo is buying their watches?!

Now text is barely readable