Take the promotion even if you feel two sizes too small for it. Most people grow into the job after they get it.

Almost nobody arrives at a job already the right size for it; the fit comes from doing the work for a few months. Feeling underqualified at the start is the standard experience, including for the people who look entirely assured. Declining on those grounds means waiting for a readiness that the job itself was going to provide.

Say yes and then be honest about what you do not yet know. Line up one person you can ask questions of without embarrassment. Give it three months before you decide how it is going.