I wonder if you're missing the quote marks or if it was a copy/paste error.

EXEC master.sys.sp_MSforeachdb

N'INSERT INTO Dba_admin.dbo.databaseDataFileSize

SELECT ''?'', SUM(size) * 8, cast(getdate() as date), cast(getdate() as time)

FROM [?].sys.database_files

WHERE type = 0;'

PS. Depending on your database sizes, you might want to cast your size column as bigint.