Recently during some internal discussions we had a lot of talk around the stack sizes for various SQL builds. Here is the summary of that talk:

 

SQL Server Build	OS Build	Stack Size
SQL Server x86 (32-bit)	x86 (32-bit)	512 KB
SQL Server x86 (32-bit)	x64 (64-bit)	768 KB (see notes 1 and 2)
SQL Server x64 (64-bit)	x64 (64-bit)	2048 KB
SQL Server IA64 (64-bit Itanium)	IA64 (64-bit Itanium)	4096 KB (see note 3)

 

Notes:

1. This (wow64 stack size) is confirmed from http://www.microsoft.com/technet/prodtechnol/sql/2005/diagandcorrecterrs.mspx: SQL Server 2000 running on X64 in WOW mode also uses more memory per worker (768 KB = 512 KB for standard stack + 256 KB used by WOW to support the stack).
2. Also check http://support.microsoft.com/kb/916950 for an issue related to SQL 2000 x86 (really this should be the only case where this configuration might be used) running in WOW64 mode on x64 OS. http://msdn.microsoft.com/en-us/library/aa384219(VS.85).aspx mentions the overhead which is required for a 32-bit process running inside WOW64.
3. Slava Oaks blogged specifically about this http://blogs.msdn.com/slavao/archive/2005/03/19/399117.aspx

I hope this summary helps someone out there!