What is the span between the thumb and the little finger called?

According to Wikipedia, "[i]n English usage the palm, or small palm, also called handbreadth or handsbreadth, was originally based on the breadth of a human hand without the thumb," as it is shown in the picture below where line #3 is the palm or small palm and line #2 is the hand.

Thus, since it seems that there there is any name for the span between the thumb and the little finger, line #4, I wonder whether, nowadays, one can call big palm that span.

For what is worth, at least to me, Merriam-Webster is not so clear in reference to how that span is called, being its palm definition "a unit of length based on the breadth or length of the hand."

Solution 1:

Line 4 is called the span or handspan.

span noun
1 the full extent of something from end to end; the amount of space that something covers:
...
(also handspan) the maximum distance between the tips of the thumb and little finger, taken as the basis of a measurement equal to 9 inches.

[ODO]

Merriam-Webster's definition of palm is based on your line 2: the breadth of the hand. Most people's palms are more-or-less square, and that distance is approximately the same as the distance from the wrist just below the ball of the thumb to the top of the palm at the base of the middle finger.