This presentation from the last open days might help with some generic hardware sizing information (e.g. at slide 9). For a couple of users any recent multi-core server / VPS with 4GB+ RAM should be enough. RAM is cheap so if your hosting doesn't give you at least 4GB for a very low price, just find a better one.

You might also consider the Odoo Online hosting, where the hardware is taken care of for you.

Hardware sizing is much more complicated than it seems. There is basically no minimum hardware specs for Odoo, the only minimum specs are software ones (PostgreSQL 9.1 for 8.0, Python 2.7, etc.). You could probably handle 2-5 users on a second-hand Pentium 4 with 512MB RAM, if you managed to meet the min software specs and were willing to take risks.

But Odoo supports very small single-user deployment and very large clustered deployment for thousands of users. And since every deployment comes with specific work patterns and performance expectations, it is quite difficult to give a magical recipe that everyone will find satisfying.