When setting up a Marketing Campaign A/B, test do sample sizes need to be 50/50?

• You are running marketing for Linkedin, and want to test Job Alert campaigns which sends recommended jobs to your users. You have the following two audiences to design an A/B test for. How would you split the test to ensure accuracy while minimizing potential negative impact of the new variant "B".
• Audiences:
  • Active users: 100,000 users each day: have returned to LinkedIn in the last 7 days
    • Receive alert every day
  • Passive users: 900,000 users (everyone not "ACtive"): Have not returned to Linkedin in last 7 days.
    • Receive alert every week.