How do I calculate the minimal order amount for a given currency pair?

I have found experimentally that there's a limit below which I can't buy or sell but it seems to be different for different currency pairs. For instance on USDT_ETH my orders hit it at 0.000003, USDT_ETC is 0.000006, USDT_XMR is 0.000002. If my buy order is below this amount I get 422 (Unprocessable Entity) error. I haven't found anything about order limits in official documentation. I want to be able to evaluate the order's amount to be correct before I place it.