There are no limits to distribution list sizes unless you are still using Microsoft Outlook 2007 without Microsoft Office Suite Service Pack 2 (SP2) or later.
If you are still using Outlook 2007 Service Pack 1 or earlier, the maximum number of names that you can include in a distribution list depends on the size of the contact files (for each contact). In general, for users of Outlook 2007 Service Pack 1 or earlier, a distribution list can contain between 50-70 names and email addresses.
2007 Microsoft Office Suite Service Pack 2 (SP2) or later removes any limitation to distribution list size. If your computer has Office 2007 SP2 or later installed, you will be able to create and use distribution lists with as many names as you want.
Note: Distribution lists created on a computer using Office 2007 SP2, which exceed the size limits of earlier versions, can only be used on computers that have Office 2007 SP2 installed. Any distribution list from earlier versions, even if previously oversized, can be used on computers using Office 2007 SP2.
For more information about distribution lists in Outlook, see Create a contact group or distribution list in Outlook.
For more information on versions earlier than Office 2007 SP2, see the Knowledge Base article on the maximum size for distribution lists