The insured amount should be based on your risk exposure, how much you might be sued for.
I disagree somewhat with the (apparent) lawyer who answered above and here's why. Your assets are irrelevant when calculating damages. But no lawyer is going to take a high damages case if there is little chance of ever collecting even if they won.
I cherry picked these two comments for reply.
I agree to a certain extent that that you should pick a limit based on your risk exposure, however, with liability coverage, there is no mathematical equation to calculate it. In my defense, I didn't suggest to purchase a limit based on how much you might be sued for, but rather, how much you might be "liable" to pay in the event of a claim. Big difference.
Your second comment mentioned damages, which is what are awarded at the conclusion of the lawsuit. At the same time, your first comment and second seem to conflict. If you should pick a limit based on your risk exposure, then clearly you must consider your assets. No?
This question comes up often, how much should I carry, and it's easy to complicate what should be easy. Lets look at it another way.
There are essentially 5 options with excess liability (I'm really simplifying it), being $1,000,000, 2,000,000, 3,000,000, 4,000,000 and 5,000,000. Sure, you can go higher, but for the sake of discussion, most people are going to fall into this range.
Putting a laborious analysis of assets and exposures to the side, which of those 5 options make you (the purchaser) most comfortable?
Assume for a second that 1,000,000 cost $300, and 5,000,000 cost $1,000 (gross estimations for sake of discussion), with the other options in the middle. How much can you afford, or are you willing to budget? Is it $500? Then buy $2,000,000, or 3.