The Upsize Wizard in MS Access works fine for upsizing most Access database, although it sometimes has issues with larger databases over 150MB. Make sure you compact and repair the database in MS Access before you do the upsize.
There is no way to tell the size of the database after the upsize, but generally you would use around 125MB to 175MB for each 100,000 posts. You should shrink the SQL Server Database after upsizing as this will greatly reduce the overall size.
Once you have upsized the Access database to SQL Server re-install Web Wiz Forums, select the database version of Web Wiz Forums you have and then select SQL Server from the list of database types. The install wizard will then set the correct settings for the type of database that you are using.