When a board game's box says that it supports two to seven players, that only means there are enough components in the box to support that many players. It does not mean that you'll have a good experience at all player counts.
Here's how you can find out what player counts a game should be played at.
BoardGameGeek is a very useful resource for board game fans. Not only can you use it to check for rules clarifications and other information, you can use it to check the recommended player counts for any game.
All you need to do is search for a game using the site's search feature and you can read up on all sorts of useful information that can help you decide if you want to buy the game.
Each game's information panel will include an official player count as well as the results of a community poll for recommended player counts. This community poll is as reliable way to check which player counts a game is suitable for.
There are currently over 100,000 board games listed on Board Game Geek, the tabletop world's equivalent of IMDB. That's a lot to choose from. Wandering into your local department store is going to give your some truly disappointing choices and browsing your friendly local game store can be overwhelming. Here are some great board games that anyone from veteran to rookie can enjoy.
For example card drafting game 7 Wonders officially supports two to seven players but the community poll results recommend playing with three to seven players and say the game is best with four players.
You can click on the player count information to view a detailed breakdown of the community poll or participate yourself.