There's probably a better way, but this should work:
select count(*)
frommaster.sys.databasesas DB
inner joinmaster.sys.master_filesas MF on MF.database_id = DB.database_id
where DB.name = 'Express_DB'
and MF.name = 'Express_Data'
and (MF. /131072.) >3.5
if @@rowcount > 0
EXEC msdb.dbo.sp_send_dbmail
@recipients = 'YourName@YourCompany.com',
@subject = 'Express Database over 3.5 Gig'