Understand the meaning of "tall order"

I was wondering why a "tall order" means a formidable task or requirement?

Is it a metaphor? If so, how shall I understand it?

An order is "a direction or commission to make, provide, or furnish something" (Dictionary.com). Tall means "large in amount or degree".

So a tall order is a direction to do something considerable. It's not a metaphor; it just includes a rarer usage of tall.

The OED says tall also means a large amount, and is originally US slang. Their first quotation is from Charles Dickens' American Notes for General Circulation (1842):

We were a pretty tall time coming that last fifteen mile.

A tall order is "something expected to be hard to achieve or fulfil" F. W. L. Adams wrote in The New Egypt (1893):

It's a tall order, but it's worth trying, isn't it?

This kind of demanding request had also been intensified with other adjectives such as big, large and strong, for example in Anthony Trollope's The way we live now (1875):

By Jove, it's a rather strong order when a girl has just run away with another man. Everybody knows it.

This use of tall is also seen in tall tale, covered by World Wide Words who begin:

Tall is one of those curious words, like nice, that has had more meanings down the centuries than you can shake a stick at. Back in Anglo-Saxon times it meant swift or prompt, and later on it variously had senses of fine, handsome, bold, strong, brave, skilful and a good fighter. It was only in the sixteenth century that it started to mean somebody or something physically higher than normal.