Hi Query King,

Please find the two Ways to get the maximum Size limit of Active Directory Attributes.

1. Using GUI.
   1. By Default, Schema Snap-In do not show in MMC Console, So it need to be register first.   
   2. Open an elevated command prompt, click Start , Type command prompt , then from Start Menu, Right Click on Command Prompt and click on Run as administrator, On the UAC prompt select Yes.
   3. Type regsvr32 schmmgmt.dll to register Schema Snap-In.
   4. You will get the "DllRegisterServer in schmmgmt.dll succeeded" as confirmation of successful Registration. 
   5. Now to Open the MMC console, Go to Run, type MMC and click on OK.
   6. it will open MMC console, Go to File --> Add/Remove Snap-in.
   7. Add "Active Directory Schema" from opened windows and click on OK.
   8. In opened windows browse, Console Root -> Active Directory Schema -> Attributes -> Select Display name.
   9. Right Click on Display Name and click on Properties.
   10. Here you can see the Size Limit information (Refer Attached Screenshot for Details.)

      2. Using PowerShell

1. Go to Start and Type "Active Directory Module for Windows PowerShell"
2. Type below command to see the size limit.

$schema =[DirectoryServices.ActiveDirectory.ActiveDirectorySchema]::GetCurrentSchema()
$schema.FindClass('user').optionalproperties | select name,rangeupper | Where-Object {$_.name -eq 'DisplayName'}

Hope it will solve your query.

Thanks Santosh, 2nd option is much easier.