You can calculate used space via dynamic management views (reference page here):
SELECT SUM(reserved_page_count)*8.0/1024
FROM sys.dm_db_partition_stats;
While I don't think you can retrieve max size, it doesn't hurt to simply set your database to the largest size, since you're only billed for space you use. Having said that: If you're only in the sub-5GB range, it's best to go with Web edition. If you're already at 10+ GB, there shouldn't be any harm setting max size to 150GB and then monitoring consumed size.
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