FEMA sets two dollar amounts each federal fiscal year. A project below the minimum threshold is not eligible at all. A project at or above the large project threshold is a large project, funded on actual costs; anything between is a small project, funded on the estimate. The amounts change every year, so use the ones for the fiscal year of your declaration.
FEMA publishes the minimum project threshold and the large project threshold for each federal fiscal year on its Per Capita Impact Indicator and Project Thresholds page. Use the amounts for the fiscal year your disaster was declared.
The size is set on the final approved amount after insurance and other reductions, so an insurance payment can turn a large project into a small one. Small projects are not trued up to actual costs, which makes an accurate estimate up front essential.