Ounces are a unit of weight. Fluid ounces are a unit of volume. Weight is the mass or "heaviness" of an ingredient. Volume is the amount of space and ingredient occupies. Your recipe is listing the ingredients in weight. If it were supposed to be volume, the recipe would say "fl. oz.".
I'm not sure if you are referring to baking chocolate of cocoa powder. 6 oz of grated baking chocolate is about 1.71 cups. 6 oz of cocoa powder is about 1.5 cups. There are websites that do volume/weight conversions for you:
http://www.gourmetsleuth.com
http://www.onlineconversion.com
I got these answers from both websites. One square is 1 oz of chocolate. So, yes, you'll need five squares.
The problem is that a lot of recipes list ingredients in weight, but not many people own a scale. The matter is confused more because not all ingredients weigh the same. For example, a cup of lead pellets and a cup of feathers both occupy 8 fl. oz. of volume. The lead will weigh a lot more than 8 ounces on a scale and the feathers will weigh a lot less than 8 ounces on a scale. Here is another example:
500 grams of mini marshmallows = 10 cups
500 grams of flour = 4 cups
500 grams of sugar = 2.5 cups
As you can see, the weights of these ingredients are the same, yet their volumes are different. This is because every ingredient has a different density. This is why you need a website to do these conversions. Another option is to look in a cookbook that has a volume/weight equivalency chart.
A lot of people are also confused about the term "dry measure". A dry measure is a measuring cup that is flat on top so that it can be leveled. A liquid measure is a measuring cup with a spout on it. Both a 1 cup dry measure and a 1 cup liquid measure have exactly the same volume: 8 fluid ounces. A lot of people mistakenly believe that a dry measure uses a different type of ounce. They'll erroneously refer to it as a "dry ounce" or a "solid ounce", or they'll refer to liquid being measured in a cup as "liquid ounces".
Don't let the term "fluid ounce" throw you off. It is simply a name for a unit of volume. It doesn't matter if the ingredient in the cup is a liquid or a solid, it is still fluid ounces.
Its unfortunate that the people who invented the English system gave the same unit name (ounce) to both volume and weight. These two types of ounces are not the same and they are not interchangeable. But there are exceptions: "A pint is a pound, the world around, for butter, eggs, milk and water". In other words, one cup of any of these ingredients will weigh about 8 ounces on a scale.