1.  Open run command by pressing Windows + R keys.
 
  1. Please close the Outlook application then Then type "regedit" on the run command. 
 
  1. Open the HKEY_CURRENT_USER
 
  1. Go to "Software".
 
  1. Then "Microsoft" Folder.
 
  1. Then "Office" folder.
 
  1. Then look for "Office 16" folder.
 
  1. Open 16.0 then Outlook > pst folder.
 
  1. Please right-click the PST folder
 
  1. New
 
  1. DWORD (32-bit) Value
 
  1. Please name it "MaxLargeFileSize"
 
  1. Right-click on it then click modify.
 
  1. Click Decimal.
 
  1. Then put the value 4294967295
 
  1. After that "OK".
 
  1. Then right-click PST again.
 
  1. New
 
  1. Same proces DWORD
 
  1. Rename WarnLargeFileSize
 
  1. Right-click on it then click modify.
 
  1. Click Decimal.
 
  1. Value 4090445042
 
  1. Then click "OK".
 
  1. After that close everything then Open Outlook.

The new max size should be 100GB