It's hard to get the total size of these logs, you may need to consider the volume of sign-ins your organization generates and the types of logs you are collecting (interactive, non-interactive, service principal, managed identity). The actual size may vary based on the frequency and variety of sign-in activities in your Azure tenant. It's also important to note that the data collected is determined by the default configurations of the log types.
In practice, you may need to export the logs to an external service like Azure Storage or Azure Monitor Logs and then use the tools available in those systems to calculate the size.