It depends on what you mean by a "really small amount" but yes, starting with small amounts and working your way up is the safest way to avoid an overdose with a drug of unknown purity like heroin. Start small.
You can overdose from snorting heroin, from smoking heroin, even from eating heroin....or from any other ROA.
-Heroin is a street drug, it is impossible to know the purity of the heroin you've got, so therefore, it is impossible to tell you an exact amount.
-Everybody reacts differently to every drug, considering personal reaction, tolerance, other drugs in your system, etc.
This question is very strange to me. Are you asking how much heroin you would need to overdose? You don't want to overdose on heroin, trust me.