Garnish: whole basil leaves
3 days ahead: Make dressing: Whisk together olive oil and next five ingredients.
2 hours ahead: Cut smaller tomatoes in half; quarter larger tomatoes. Set aside. Drizzle dressing over tomatoes. Sprinkle with basil. Let stand at least 15 minutes and up to 1 hour.
Just before serving: Drain excess liquid. Garnish with fresh basil leaves; serve at room temperature.