Is Morgoth, the original enemy in J. R. R. Tolkien's Middle-earth ever visually described? I envisage him as a bigger, meaner version of the physical Sauron, but I cannot find any reference in The Silmarillion.
Depends. He takes on various forms at different times. He's mostly described as a huge dark man and this is the form he takes and is locked into after his first imprisonment.
His first appearance during the creation and the First War was
"a mountain that wades in the sea and has its head above the clouds and is clad in ice and crowned with smoke and fire; and the light of the eyes of Melkor was like a flame that withers with heat and pierces with a deadly cold."
He lost his foot and suffered various un-healable wounds after his combat with Fingolfin and burnt his hands stealing and holding the silmarils.
Melkor was grandiose. His helmet, as the Morgoth, was so heavy Beren could not lift it [Lays of Belereiand, 8.4136-4137], and he had a scar and limp [12.3604-09, 3615-3617, 3632-3634]. Melkor's physical form in the beginning is described as:
because of his mood and the malice that burned in him that form was dark and terrible. And he descended upon Arda in power and majesty greater than any other of the Valar, as a mountain that wades in the sea and has it head above the clouds and is clad in ice and crowned with smoke and fire; and the light of the eyes of Melkor was like a flame that withers with heat and pierces with deadly cold. [Silmarillion; Ainulindalë]
He is also said to have walked around in various "shapes of power and fear" [Of the Beginning of Days]
he put on again the form that he had worn as the tyrant of Utumno: a dark Lord, tall and terrible. In that form he remained ever after. [Of the Darkening of Valinor]
Don't over-estimate how big Melkor, or any of the Ainu were. There is also another description of him when he battles Fingolfin:
Morgoth came, climbing slowly from his subterranean throne, and the rumour of his feet was like thunder underground. And he issued forth clad in black armour; and he stood before the King like a tower, iron-crowned, and his vast shield, sable unblazoned, cast a shadow over him like a stormcloud...
Melkor held aloft Grond, the Hammer of the Underworld, and swung it down like a bolt of thunder... Grond rent a mighty pit in the earth, whence smoke and fire darted...
Morgoth bore his shield upon him. Thrice he was crushed to his knees... Morgoth set his left foot upon his neck, and the weight of it was like a fallen hill. Yet with his last and desperate stroke Fingolfin hewed the foot with Ringil [The Ruin of Beleriand]
Morgoth is big but not like a giant. When he put his foot on Fingolfin's neck he could still use his arm to swing his sword.
Morgoth can take on different evil forms at will, but his true appearance is an invisible cloud of dread that sucks out the color and light from its surroundings
In 1967, Tolkien wrote some linguistic notes about the elvish root √PHAN- "cover, screen, veil", from which he derived the elvish word fanar, meaning the physical forms adopted by the Valar and Maiar. He then digressed for a few pages to discuss these forms. These notes were published in Parma Eldalamberon #17, and then later republished in The Nature of Middle-earth.
During these notes Tolkien discusses a bit what Valar were like when not "clad" in their fanar.
Melkor, they said, was invisible, and his presence was revealed only by great dread and by a darkness that dimmed or blotted out the light and hues of all things near him. The Maiar corrupted by him stank. For this reason neither he nor any of the evil Maiar ever approached one of the Eldar that they wished to persuade or deceive except clad in their fanar. These they could still make to appear beautiful to Elvish eyes, if they wished — until after the great treachery of Melkor and the destruction of the Trees. After that Melkor (Morgoth) and his servants were perceived as forms of evil and enemies undisguised.
The Nature of Middle-earth - "The Visible Forms of the Valar and Maiar"
Initially he could take any form at will, but upon becoming evil he chose a form that is said to be "a mountain that wades in the sea, and has its head above the clouds, and is clad with ice and crowned with smoke and fire, and the light of the eyes of Melkor was like a flame that whithers with heat and pierces with a deadly cold", and The Silmarillion stated that "he stood over the king as a tower...and...cast a shadow over him like a storm cloud".
...a mountain that wades in the sea, and has its head above the clouds, and is clad with ice and crowned with smoke and fire, and the light of the eyes of Melkor was like a flame that whithers with heat and pierces with a deadly cold. —The Silmarillion, Ainulindalë
He stood over the king as a tower...and...cast a shadow over him like a storm cloud. —Quenta Silmarillion, "Of the Ruin of Beleriand and the Fall of Fingolfin"
After stealing the Silmarils, he used them as his crown jewels, so they shone on his head. There are illustrations of him that depict his appearance:
Also, Sauron's armor in the movies was based on Morgoth's in the illustrations: https://lotr.fandom.com/wiki/Sauron#The_Lord_of_the_Rings_film_trilogy
He also wears black armor and a helmet similar to Morgoth's in The Silmarillion, and wields a giant mace (perhaps based on Grond) capable of killing scores of men in one hit.
So I guess Morgoth looks like a gigantic movie!Sauron with three Silmarils on his helmet's forehead.