Sample size calculation for a marker to improve diagnosis

Hi!

I am trying to calculate sample size for my study. I am interested in using more sensitive marker to diagnose an outcome faster and more precisely. The incidence of given outcome diagnosed with currently used marker is ~15% and I expect it to be higher (but I can not reliably predict how much higher).

The goal of my study is twofold:

  1. Assess the incidence of outcome using new biomarker
  2. Measure some biomarkers with potential predictive value for the outcome

What would be an optimal way to calculate sample size for this study?

I would be very grateful for any help.