I have two hot and one white(neutral) coming in, 100amp, 240v. I want to add a laundry room and need a subpanel. The only 240v 30A I need is for the new dryer. The rest is 120v for two floresant lights and one ventilation fan and two plug. Would 50A be good? Or do I need to increase the amps in the main service to 200amp?

Okay, please bare with me here. I am concerned about your experience level with electricity. Adding a sub-panel is not a simple task. For one thing, you have to work inside the main panel and even if you shut the panel down via the main breaker you can still manage to kill yourself if you hit the wrong terminals.

50 amps would be fine for just a laundry room. The part I am concerned about is whether you know enough about electrical code requirements to do the job correctly. Things like ground/ neutral isolation, cable types, grounding and bonding, box fill, GFCIs and the like. If you are confident you can do a code compliant job, then OK. However if your main panel is full to the point of needing a subpanel, you might want to think through future expansion and adequacy of your existing service for the long term. It may be worth it just to upgrade the service and install a larger main panel.

I will have to assume that you always had a washer and dryer. Both of these are to be figured in when sizing a service, and should have been included into your existing 100 Amp service.

Adding a subpanel should be fine for these new circuits since, I assume, they were already connected.

Back to you, did you already have the washer and dryer, or are they new.

Thank you for answering my question. I have been taught how to run wire and did have some schooling about electricity.

Thank you for your concern. I will be very careful. In case I could not manage it, I will hire a electrician.

Only the dryer was included in the main panel. All other items are added on new.