Request an amount large enough to show you the buyer has financial resources. You do not want to waste your time on someone with little or no cash and high hopes of finding financing.
In addition, think about the hostage value of the earnest money. A small amount could be pocket change, a buyer might abandon the earnest money and elect to buy something else that seems more desirable or even less expensive. One of my clients once accepted $500 earnest money on a $198,000 home.
Thirty days later, on the day before closing, the buyer advised that she would not proceed to purchase. She had found another house for only $187,900 and was all too happy to breach her purchase contract with my client and lose her $500.