Some questions about what has been posted on the site

Dear Rabbis!  I have a few questions about the answers posted on the site recently. 1. There is some confusion about hanging a mezuzah (and a contradiction between the answers). A room of what size must have a mezuzah, four square cubits, which is two by two cubits, or four by four cubits, which is 16 square cubits? And what if the room is one cubit long by twenty cubits wide, must it have a mezuzah or not? 2. The Rabbis were asked: "How can a grill of an oven that was used for meat, be prepared for use in a dairy oven?" and the rabbis answered: "You can place it in the oven when it is empty, and turn it on the highest heat for about an hour." Does this mean that it is permissible to kasher meat utensils for use with milk lechatchilah (ab initio)? 3. The Rabbis wrote: "The blessing on fragrant substances is divided into three blessings, “Who creates fragrant trees”, “Who creates fragrant herbs” and “Who creates various species of fragrances”. And each of them does not apply to a species whose blessing is different. Therefore, if the correct blessing is “Who creates various species of fragrances”, is one says “Who creates fragrant herbs” on spearmint, this will not accomplish anything." But it is explicit in the Shulchan Oruch that “Who creates various species of fragrances” applies to all species. (And besides, there are five blessings, not three.) (And if the material is really a matter of doubt, he has no problem with “not having learned.")    

1. The minimal size of a room that obligates one to affix a mezuzah is four by four cubits, that is, sixteen square cubits. In this case mezuzah has to be affixed with a blessing. If the area of a room is sixteen square cubits, but its shape is not four by four, there is a dispute between the poskim and therefore a mezuzah hast to be affixed without a blessing.

2. Indeed, it is forbidden to convert utensils from meat to dear and vice versa ab initio. However, the question was asked about a man who bought an oven and wanted to convert it for a different use. In this case he’s allowed to determine what he wanted.

3. One has to recite the proper blessing over every species, and it is forbidden to recite “Who creates various species of fragrances” on species whose blessing is different, even though it does apply retroactively. It’s the same as making a blessing “everything came into being by His word” over different foods: although it applies retroactively, one has to find out what the correct blessing is, and it is forbidden to rely on the blessing for blessing over every type of food.