There are no rules dictating how big a transaction log should be. For a new database, a log size of around 20 percent of the overall database size is a good starting point. The actual size required depends on how the database will be used. Factors that affect size requirements include:
Because there are many factors involved in transaction logging, you cannot accurately predetermine how much log space a particular database will require. The best way to estimate size requirements is to simulate the production environment as closely as possible (for example, by running applications with the same number of users as you expect to be using the database in the production environment).