Don't change it in MySQL as it will be out of sync with vardefs.
Instead create an Extension that changes the length, then a Quick Repair and Rebuild will update it in the database.
Create a file <sugar_root>/custom/Extension/modules/Tasks/Ext/Vardefs/custom.php with the following contents:
You can change the length to any value you like. Create any files in the path if needed.
$dictionary['Task']['fields']['name']['len'] = '255';
Then do a Quick Repair and Rebuild from the Admin Panel. Make sure you scroll all the way to the bottom of the results page and look for an Alter Table query. Sometimes you have to click the Execute button to complete the modification of the MySQL table to be in sync with vardefs.