While sizing one of the points which comes to the mind is how can we SIZE our Azure SQL Database. Azure SQL Database tier is based on the unit known as DTU. This is combination of few things. Please refer the official Azure documentation for details. However Microsoft Azure also provides a small utility to determine the right DTU you may need.
Azure SQL Database DTU Calculator is THE tool an architect should keep handy available http://dtucalculator.azurewebsites.net/