SQL Server - Space Available
From looking at a Profiler trace of the SSMS dialog opening the queries are essentially:

Size:

SELECT sum(df.size) * 8 / 1024.0 Size_MB
FROM sys.database_files df;


and

Space Available:

with q as
(
    SELECT
    (SELECT SUM(CAST(df.size as float)) FROM sys.database_files AS df WHERE df.type in ( 0, 2, 4 ) ) AS [DbSize],
    (SUM(a.total_pages) + (SELECT ISNULL(SUM(CAST(df.size as bigint)), 0) FROM sys.database_files AS df WHERE df.type = 2 )) AS [SpaceUsed]
    FROM sys.partitions p 
    join sys.allocation_units a 
      on p.partition_id = a.container_id 
    left join sys.internal_tables it 
      on p.object_id = it.object_id
) 
select (DbSize - SpaceUsed) * 8 / 1024.0 SpaceAvailable_MB
from q;


So Size includes the log file, but Space Available does not.  It's only the unallocated space in database files.