Dear Ali Awan,
For each field in Dynamics AX we have a extended data type also. You can open the dynamics Development workspace using Ctrl+Shift+W keys and open the AOT. In AOT go to data dictionary and select Extended Data type and search JournalDescription or Description and change the string size to 200. If the description is changed all the description field throughout the AX will be changed to 200. For journal only change JournalDescription. Hope it will solve your issue.
Omer Awan