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What would be a clear and concise way to describe someone whose ambitions or aspirations far exceed his means or abilities?

Thanks in advance

A person with ambitions or aspirations that far exceed their means or abilities would be quixotic. The term is a good fit because of its definition and because it brings to mind Don Quixote providing a mental picture of aspirations exceeding abilities.

quixotic

ADJECTIVE

Extremely idealistic; unrealistic and impractical.

‘a vast and perhaps quixotic project’ English Oxford Living Dictionaries

In this case an example would be: "Bob has quixotic ambitions"

Etymology

Quixotic Has Roots in Literature

If you guessed that quixotic has something to do with Don Quixote, you're absolutely right. The hero of the 17th-century Spanish novel El Ingenioso Hidalgo Don Quijote de la Mancha (by Miguel de Cervantes) didn't change the world by tilting at windmills, but he did leave a linguistic legacy in English. The adjective quixotic is based on his name and has been used to describe unrealistic idealists since at least the early 18th century. Merriam-Webster

I think megalomaniac comes close to what you are referring to:

someone who has an unnaturally strong wish for power and control, or thinks that they are much more important and powerful than they really are.

(Cambridge Dictionary)

They're a dreamer

dream·er (n)

a person who dreams or is dreaming.

a person who is unpractical or idealistic. "a rebellious young dreamer"

synonyms: fantasist · fantasizer · daydreamer · romantic · sentimental(ist)

Your exchange with @Edwin Ashworth makes it seem more along the lines of egocentric (disconnect from reality). As far as hopeless, you could describe them or the situation as classically pathetic.

path·et·ic (adj)

marked by sorrow or melancholy : sad

pitifully inferior or inadequate "the restaurant's pathetic service"

You could work in an allusion to Thurber's character Walter Mitty.

Wikipedia explains:

Walter Mitty is a fictional character in James Thurber's first short
story "The Secret Life of Walter Mitty", first published in The New
Yorker on March 18, 1939, and in book form in My World and Welcome
to It in 1942. Thurber loosely based the character, a daydreamer, on
his friend Walter Mithoff. It was made into a film in 1947 starring
Danny Kaye, with a remake directed by, and starring Ben Stiller
released in 2013.The character's name has come into more general use
to refer to an ineffectual dreamer and appears in several
dictionaries. The American Heritage Dictionary [see also the American Heritage Dictionary of Idioms] defines 'a Walter
Mitty' as "an ordinary often ineffectual person who indulges in
fantastic daydreams of personal triumphs".

Examples:

He has been described as the Walter Mitty of the political world, a
complete nobody who has somehow contrived a career out of standing on
a soapbox and protesting against anything the mainstream politicians
do.

...............

My father worked for the same company for over 50 years and never even
left his home state, but he was always something of a Walter Mitty,
dreaming about a life of adventure.

[both Farlex Dictionary of Idioms]

Overreach is a somewhat bland term for this. Collins:

If you say that someone overreaches themselves, you mean that they
fail at something because they are trying to do more than they are
able to.

“His reach exceeds his grasp”—but it comes from a Robert Frost poem with the line “A man’s reach should exceed his grasp.”