There will be four things to consider when taking that construction loan. You should be able to determine what you need when you consider what the design of the home is, where it is being built, the actual cost of the construction and the mortgage payment.
Get several estimates on the cost of the construction you are undertaking. Most banks and lenders will want detailed estimates however they will also provide a cushion or additional amount to the loan to cover the possible increase in the cost of materials, labor or other aspects of the project.