The beaches in Monterosso weren't enormous, but not tiny either. Some were private and some were public. As I'm sure you can imagine, the public beaches were more crowded than the private sections, but since I was there in early/mid May they weren't very crowded in general. In my opinion you could spend a day in Monterosso seeing the different beaches, having a nice lunch, and a drink at the end of the day.

I will have to agree with staying in Monterosso! And 3 nights would probably be enough. That's how long I stayed a couple years ago. Be sure and pick up the Cinque Terre card at the train station in LaSpezia. This will get you on all the trails in the "park" as well as the train. There are several other hikes you can take other than the popular hiking trail between villages. Ahhhh Cinque Terre, I absolutely adored my time there. There was a little cafe hanging off the cliff right before you got to Riomaggiore on the trail with a great view overlooking the rocks, water and the town. Just take the trail from Monterosso to Riomaggiore. A short and easy 20 minute walk.

What other places can you buy the Cinque Terre card ?

Marko, you can get the Cinque Terre card at the little train stations in each of the five villages. Here is the link for info http://www.cinqueterre.com/eng/information/card/ Oh, and I was having a senior moment yesterday! I stayed in Manarola, NOT Monterossa. Manarola is the second little village if you start from Riomaggiore.