Hav you tried Larger - 200% or Largest - 250% ?
Please post your replies here in the forum.
Are you sure you are running Lightroom 6, and not an older version? 250% was added in LR6, and I'm pretty sure LR5 had 200%.
Go to Help > About Lightroom, and check the version number.
Thanks, I checked as yu suggested. It is Adobe Photoshop Lightroom CC 2015.1.1 release; Camera Raw 9.1.1.
It does not have the option other than SMALL or LARGE.
I have not updated it to the latest version as it is supposed have some issues with crashing, import, et. I will updated it when the issues are resolved, hopefully, shortly.
As a first step, try resetting the preferences.
If that doesn't fix the problem, I would unistall/reinstall. As you don't want the 6.2 version, see this page:
Thanks, yu have been helpful. I will wait till a new update of LR6 is available to change the font size.