You can explore this using a loop of string for your fixed length of wall. Starting with (an admittedly very useless) room with just two corners, each time you add in another corner (going from triangular, to square, to pentagonal, etc), you increase the area the string encloses. If you carry on adding corners you shape becomes more and more like a circle, and it is a circle which encloses the most area for fixed perimeter. (If you’ve got a mathematical bent why not try deducing the area of these shapes, and prove that it increases as the number of sides increases. You can check you calculations for the areas with those listed on Wikipedia)