Magen Avraham 32:1: צריך לכתוב ד' דאחד כל כך גדולה כמו ד' דלתי"ן קטני' (כתבי האר"י).
The dalet of echad should be large the size of 4 small daletin (in the name of Ari-zal).
The MB 32:1 (in name of Pri Megadim): maybe this is not estimated by the script actualy used, rather if it has the size of 4 tiny dalet, it is sufficient. And therefore the custom is to write this dalet only larger than a regular dalet.
This means that the custom wasn't to make the large dalet actualy 4 times a dalet in the script, only slightly larger.
R. Menahem Dovidovitz shlita holds that the proper writing of this dalet (4 x dalet ktana) is done by doubling the size of the rosh to its height and twice its length, so it is equal in area to 4 regular dalets.
My opinion is that this is not the meaning of the MA (Ari). The extending of the gag twice is not considered doubling the dalet, since the dalet once [its gag] becoming thicker must be also extended in order not to distort the tzura [this is actualy a regular big letter, like in sefer tora that is custom to making its features twice the size of the regular ois] . Also, this doesn't explain "4 small dalet".
The true meaning of the MA is 4 times the height. But what is the meaning of "4 small dalet"? why not "4 regular dalet"??
The meaning of a 'dalet ktana' is, that mentioned in the hallacha - if the foot of the dalet was short (see SA Harav 32:19), if it has mlo ois ketana [a yuds height] it is still kosher. This is a dalet ktana, another example is the upside down dalet of the alef, it is considered a dalet ktana hafucha. The size being 1k at the gag and 1k at the foot. The total height 2k.
So a regular dalet is 3k high, a small dalet 2k high. There is no consideration of the length of the gag in these dalets.
The big dalet of echad is 4 times a small dalet, which means the gag is 4k thick, and the foot is 4k long (being equal to the hight of gag) total 8k high. I would think this is also a hallachic requirement, since the foot should be mlo ois ketana, and this specific gag is 4k, so the foot should be its own measure of MOK, ie equal to the gag (at least lechatchila).