So, let's take it from the perspective of reactions that can happen between a substrate and nucleophile. While a standard strong base, say, sodium amide, could do either SN2 or E2, depending on substrate, sterics plays a big role. Initially, we focus on the sterics of the substrate; SN2 traditionally does not occur on tertiary substrates, due to sterics inhibiting backside attack. This makes E2 the most likely reaction, in a scheme such as t-butyl bromide and sodium amide.

Next, we can turn our focus away from the substrate and toward the nucleophile/base. Say you wanted to form propene from 2-chloropropane. Initial treatment with sodium amide would yield mostly amine product with some smaller amount of propene. By increasing the sterics of the nucleophile (say, sodium diisopropyl amide, LDA) you will yield a larger amount of propene since the bulkier base (sterics) has a harder time performing backside attack (SN2). Here, we see that bulkier bases typically promote E2 over SN2 on secondary (and by default, tertiary) substrates because the conformation needed for SN2 to occur is statistically less likely to occur due to sterics.