Preachers who are more accustomed to preaching from the New Testament are likely to choose too short a preaching text when attempting to preach from the Old Testament. As a general rule, with obvious exceptions, preaching texts ought to be longer in the Old Testament than in the New Testament, especially the epistles. The reason for this is that much of the Old Testament is written in narrative form. Therefore, a broader context is required to make sense of any given verse. Contrast this with the epistles, which are packed so densely that a single verse can keep a preacher occupied for weeks. This is because the writer of the epistle presupposes a wealth of Old Testament Scripture in the tight and complicated statements that he makes. In the epistles, the train is already moving at full speed, so to speak. On the other hand, the train is still picking up speed through much of the Old Testament. Allusions and Scriptural references gain momentum as revelation progresses and statements are impregnated with earlier passages. If we try to preach from a single verse in Genesis, for example, we are likely to be frustrated. If, however, we tackle a chapter at a time, or sometimes two, amazing revelation begins to emerge from the page.
The inverse is also true. It is easy to select too long a preaching text in the New Testament, especially the epistles. Each clause in the epistles is packed with theology and consequence. Often a single verse will have many long and tangled roots into the Old Testament, which ought to be explored one at a time. I once heard a fabulous preacher named John Lennox, who was speaking on behalf of Ravi Zacharias International Ministries, elucidate a profound truth about the New Testament. He explained to us that the New Testament is often short on illustrations, not so that the preacher can find and use stories from his own life, but because the illustrations are already written down by God in the Old Testament. Good preaching from the New Testament will slow down, selecting shorter preaching texts, so that these Old Testament illustrations can be located and brought to light.
It is not a cardinal rule that preaching texts from the Old Testament must be long and preaching texts from the New Testament must be short. This may be a general pattern, but there are far too many exceptions to make this point too severely. What matters most is that no preaching text, either from the Old or from the New, is too short or too long. Context will have to be your guide when making this decision.