Jimmy Olsen had a few giant issues during his publication, and had a few opportunities where he turned into a giant!
Here (though one cover doesn't look it), is a couple of times Jimmy was truly larger than life!
The Human Skyscraper
First up, from Superman's Pal Jimmy Olsen #28 (April, 1958) by Otto Binder, Curt Swan and Ray Burnley (under an unrelated cover by Curt Swan and Stan Kaye), Jimmy goes ape over Metropolis after growing to giant-sized!
Jimmy has a toothache, so, instead of going to a dentist, he goes to see inventor Professor Potter, who shows off his new growing formula. When Jimmy's tooth bothers him, the Professor gives him a pill for the pain. Jimmy takes off in the Flying Newsroom, looking for a story as a foreign dignitary comes in by ocean liner, but a storm gets Jimmy to crash the helicopter on an island instead.
Jimmy awakes, to find himself bound and naked on the beach, having grown to Gulliver sized! Jimmy calls out to a passing ship, but triggers an avalanche, knocking him unconscious. The ship crew hear his call, and its captain, Wonders Wilson, trades with the natives to take the large comatose man back to civilization.
Planning to show off his King (Kong)-sized wonder, Wilson sets up Olsen on display (who awakes, but is dazed and childlike due to a blow on his head, and goes ape, though without being furry like a time when he became a gorilla). Jimmy breaks free of his bonds, while Clark Kent finds out about Jimmy's condition, and checks with Professor Potter (who realizes he must have given Jimmy the growing formula, and looks for a cure).
Superman distracts Jimmy to prevent him from destroying (most) of Metropolis, and realizes, as Jimmy climbs a building with a spire (to get the needle on top to poke at his aching tooth), that Jimmy has a tooth needed to be pulled. After getting Jimmy some anesthetic, Superman has the tooth pulled (a good thing too, as the gas also happened, coincidentally!!!, to cure Jimmy's gigantism!). Superman retrieves the damaged helicopter as Jimmy promises to see a dentist regularly twice a year from now on!
The Giant Turtle Man
Jimmy's other giant transformation (covered here) is one of his most famous (or is that infamous?), the Giant Turtle Man evolution from Superman's Pal Jimmy Olsen #53 (June, 1961) under a Curt Swan and Stan Kaye cover, with a story by Jerry Siegel, Curt Swan and John Forte.
This time around, Jimmy, Lois Lane and Clark Kent are on an ocean cruise, where Jimmy wanders off on a beach, and finds a trunk full of advanced gadgets, and, trying one, finds it can enlarge living creatures. Lois convinces him to lock it up, and bring it back to the ship, where he planned to show Clark (but, Clark got lost on the island, and stayed behind, missing the ship leaving...an excuse to allow him to head off to a long deep space mission).
Jimmy accidentally triggers his signal watch, so Superman shows up before leaving for space, and warns Jimmy NOT to fool around with items in the chest as he heads into space.
Of course, as Lois shows Jimmy things she got, including a pet turtle, Jimmy accidentally triggers the growth ray, and, as it strikes him and the turtle, they both grow, and Jimmy also takes on some turtle traits. As a Giant Turtle Man, Jimmy attacks a local bridge, tearing it apart, and taking the pieces to stuff in a volcano. He does the same with a few submarines (using transatlantic cable to rope them up). Superman returns from space, and stops Jimmy after Lois fills him in.
Then, Superman gets a telepathic message from his old college girlfriend, mermaid Lori Lemaris, who informs Superman that Jimmy's growth and menace came from Atlantean Goxo, who was a criminal scientist, who was to be exiled, and put together a trunk of weapons for someone to find, so that someone would grow, and he could take telepathic control of them (to fill a volcano on an island, to stop a group of scientists from finding the pirate treasure on the island). After shrinking Jimmy with Brainiac's shrinking ray (which also took out Jimmy's "turtleness") Superman and Jimmy go to retrieve the pirate booty from the island!
This Jimmy Olsen transformation would return many times (as others of his did), but editor Mort Weisinger might have been a little biased towards this one, and he used the idea in his Thrilling Wonder Stories anthology as well. Later Superman writers would use it too, first as a pizza pitchman for a new job for Jimmy as he was away from the Daily Planet for a time, then in the Countdown To The Final Crisis as well. Thankfully, the Brave and the Bold cartoon this back for fun (and many other of Jimmy's changes as well).
Jimmy wouldn't get to be a giant much in later years (though he gave it a shot with borrowing Legion of Super-Heroes member Colossal Boy's uniform for an issue), but, as you can see here, Giant Jimmy Olsen issues were a popular thing, even giving Jimmy's human skyscraper a cover feature!