Commentary for Genesis 6:15

THE LENGTH OF THE ARK SHALL BE THREE HUNDRED CUBITS. The verse specifies for us measure of its length, width and height...to let us know the greatness of the miracle, that a small space was able to hold much, because there were large creatures there such as elephants and wild oxen.

וזה אשר תעשה אותה, the measurements and shape are introduced by the word זה. The reason why the Torah uses the masculine mode זה instead of the feminine mode זאת, seeing that the word תבה, ark, is feminine, is because the word זה, “this,” masculine, describes the שעור, measurement, a noun which is masculine. G’d commanded Noach to make the width of the ark one sixth of its length, and the height one tenth of its length.] According to Bereshit Rabbah 31,10 the reason that the Torah gave these dimensions is precisely that shipbuilders are advised to stick to the relative measurements of width, length and height recorded here.

And you shall do this... this Occurs three times like here another time "and you shall do this on the altar" (Exodus 29:38) and to say that just as the altar atones, so you should build an altar to atone for when you come out.