All Azure SQL databases are identical in functionality.
(Excluding items such as Secondary Reserve, Restore Thresholds, Uptime Service Level Agreement, and other requirements).
The differences in Azure sql db will be the number of resources you will get, the maximum number of simultaneous connections, the maximum number of sessions, ….
It all depends on your pricing levels, maximum data size, and DTUs.
Price levels are:
Premium level and
Basic the level is probably what you are looking for.
The disadvantage is that it only goes up to 2 GB.
This is not free as the SQL Server Express Edition, but it will be the lowest cost, as far as you can stay below 2 GB.
If you need more space than 2 GB, you will need to get the
Standard level, it will be a bit more expensive depending on the number of
DTU you will need. You can go up to
250 GB in this level.