No space for "Description" of photos??

As Old Toad says, a screen shot would let us know just what you mean.

The "Description" you describe is called the Caption in the Info Window, and you can put lots of information in there. In my screenshot you can see that I have included Bio information about my wife's grandmother. There's a good bit more but, as you said, you can't see much of it without scrolling.

However, you can print a "Contact Sheet" with one or several images that includes the full caption along with date, keywords, title, or whatever you like from the info window (though not location, apparently.)

This doesn't have to be printed on paper-- you can print it to a PDF file having many pages of pictures with descriptions.

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

What system and Photos versions are you running?

Where is the library located? If on an external drive how is the drive formatted and connected to your Mac?

Can you post a screenshot of the Info pane you're getting in Photos?

First give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

The field "Description" has been renamed to "Caption" in the Info, when macOS 11 came out, see: Add titles, captions and more to photos using Photos on Mac – Apple Support (UK)

In macOS 10.15 it used to be called "Description". Add titles, descriptions and more to photos using Photos on Mac – Apple Support (UK)

Or are you trying to add descriptions to photos in Shared Albums? In Shared albums it is not possible to add captions or descriptions. You can only comment on a photo.

Cynthia Cheney wrote:

new Caption (formerly Description) space often appears on my screen with no label,

That happens to me, as well. But when I click on that area, I find that it is filled with blank spaces! In this screen shot, I've selected the spaces:

I have no idea where they came from, but I just looked, and I have a library with over 2000 of these. I'm thinking it's time to erase all those. (I fear that most of these are not spaces, but rather they contain invisible characters of some sort with some mysterious meaning. A message from aliens, perhaps? Do I dare erase them?)

You might try increasing the contrast of your screen in the System/Accessability/Display preference pane.

I use the dark mode for eye reasons and get this:

Oddly, it is still called "Description" in Print>Contact Sheets>Captions. "Description" is one part of "Caption," there...