There is no such thing as SQL Server professional edition. There is Express, Workgroup, Standard, Enterprise and Data Center (SQL 2008 R2 and up).
Express is the only edition which has a size limit. SQL 2005 had a 4 Gig database size limit, SQL 2008 R2 changed that to 10 Gigs. For all the other editions the sizes are based on the amount of disk space you have.
The indexes (both clustered and non-clustered) are B-Tree indexes. XML indexes are also B-Tree indexes under the covers. Full text and spacial are totally different (I'm not actually sure what they are using).
There are plenty of databases will multi-billion row tables and multi-TB database sizes.
Also keep in mind that SQL Server 2008 and SQL Server 2008 R2 are actually two totally different versions even though the marketing names appear to be the same.