In Nintex workflow we can define two types of variables like Single Line Text variables and Multiple line variables, Is there any limitation on number of characters that we can store in these fields ?
I'm not 100% sure about o365 but I presume it follows the same limitations as onPrem. Detailed information about column limits can be found here: Software boundaries and limits for SharePoint 2013
long story short: Single Line Text - 255 characters, Multi Line Text - 1,073,741,823
Be aware that if you use this information as part of an url you have even stricter limitations. A part of an url (between two "/"), for example the name of a folder, can only be 128 chars.
Single line is 100% over 255, as I have used it multiple times to create huge csv files.
and in regards to Multiple Line, I doubt very much you would hit the limit.
I found limits that prevented me from generating Word and Excel documents in a document library from another list.
The list item multiple line field had no limits and the Nintex workflow could take all of that text and place it in my Document library.
The issue was with the document library itself, the multiple text field, I had to tick "allow unlimited length in document libraries" in the fields settings and all worked fine.
In short I thought a limitation of updating fields via Nintex was causing an issue. Turns out Nintex wasn't the reason it was the Document Library. Might be useful information at some point for someone reading this page.