DB Recommendation While Minimizing Size
Short answer: It depends on a lot of different things.

Longer answer w/ questions: If you haven't used any database management systems before, then any of the main options (SQL Server, Oracle, PostgreSQL) will suit your needs. However, do you already have a DBMS installed on a server, or is this a personal project starting from scratch? Do you require a physical server and your own copy of the software, or are you willing to explore cloud-based options?

What are your plans for backup and restoration of critical data? What level of availability do you need (for example, 5 9s)? Apart from loading data every 30 seconds, are you querying this data often or at all, or are you simply storing it?

Without knowing anything else about your application, how it's going to be sending data to this central DB, and how you're going to use the data, it's hard to make a firm recommendation (at least for me).

As for data types and minimising space used, you've got the right idea. Good indexing strategies based on a use case come into play when trying to minimise data usage. Minimising data duplication (look at normalisation forms if you're not sure about it) using properly set up Dimension and Fact tables in the appropriate layout (Star, Snowflake etc.), assuming a Data Warehouse use (that's sort of what it sounds like) will help with query performance as well as minimising space used.

TL;DR: Just spin up an instance or two of Microsoft SQL Server on Microsoft Azure to test your application/service and see how it goes. It's scalable, can be well within a small budget and if you don't need it, you can get rid of it.

http://azure.microsoft.com/en-us/services/sql-database/