Look closely at the trip itinerary. Some trips go nonstop from morning until late evening, while others are more loosely scheduled. Which is best depends on your needs. You may want to pack in plenty of activity, or prefer free time to explore on your own. Generally, the smaller, the better.
Large groups tend to overwhelm small destinations.
Smaller groups are more able to interact with the local population. Again, this depends on your own preferences. Some trips, such as those organized by Elderhostel and ElderTreks, are specifically aimed at older travelers. But you may enjoy a trip that attracts a variety of ages. If you like the idea of a new place each day, but hate to unpack all the time, opt for a ship cruise.