One does not make a Brocho on every room that requires a Mezuzah. There are a number of conditions in order to be able to make a Brocho on the Mezuzah. Although we are often stringent and follow the opinions that the room requires a Mezuzah, in order to make a blessing, we have to be able to satisfy all proper requirements so as not to risk the serious Aveirah of making a Brocho Levatalah, a blessing in vain. The rule is that anytime a Brocho may not be required, we do not make it. For example, many people make a Brocho on the front entrance or the main entry into the home. Usually, the first room - the foyer - is less than Dalet al Dalet, less than 4 x 4 Amos, which according to many opinions does not require a Brocho. Although this is an entry room and many Poskim say it therefore does need a Mezuzah, nevertheless, one should not make a Brocho unless it is a room that according to virtually all Poskim requires a Mezuzah. Rather, one should make a Brocho on a Mezuzah in a room that definitely needs a Mezuzah according to all opinions, even if it is not the main entrance or an entrance from the outside, but inside the house, and have in mind all the Mezuzos in the house while reciting the Brocho