pfSense has a 'GUI'.
That's a complicated word for : it' s a web server, which means the start of the answer to your question is here :

Here is the file - line 197 and 199.

You might think : that's easy, change the two '16' to 'bigger then 16' and you're done. You have a good chance that this is true.
Still, you have to check things : where and how is it saved - what will breal elsewhere as the size in implicit or not; known to be '16 chars max'. ?
The final result will be present, of course, in the config.xml file, so start checking that the new name length really is saved. Check also how it is saved : is something else enforcing the '16' char size for 'some, yet to be determined reason'.
Check also other, related files, you know now where they are, as the name of the group is used on other web pages = php (and other) files. 16 chars wide can be enforced there also.

In short : yes, you can change it, but be ware of the side effects, known, or unknown for the moment.

Also, when upgrading, the "system_groupmanager.php" will get overwritten with the new official version.
So, instead of editing this file, create your own "patch file" and add it in System > Patches.

edit :

all goes well ?