The smallest useful next step is the easiest real action that moves the goal forward or reduces uncertainty.
It might be finding one balance, asking one question, changing one item in a scenario or putting a date in the calendar. It does not need to solve the whole problem.
Make it startable
“Sort out retirement” is too large. “Find my latest retirement account statement” is startable. “Move house” is too large. “Write down the three locations we would seriously consider” is startable.
Worth knowing: When progress stalls, the next step is often still too big. Shrink it without shame.