Why does the diameter of the bullet matter?
I mean, what would happen if I picked up a 90mm, 1kg rock and threw it at someone wearing a personal shield?
A shield that can be overloaded simply by throwing a rock at it would be pretty useless.
I think you'd be better off by basing the threshold on a bullet's kinetic energy, rather than on its diameter.
For reference, the Wikipedia page on shotgun slugs lists kinetic energies in the range of 2000-18000 J. This is probably about what you could expect from a large-caliber gun that can easily be carried and fired by one person. Meanwhile, the Anzio 20mm anti-materiel rifle fires a 20mm bullet with 65000 J of kinetic energy. This is probably close to the upper end of what a man-portable firearm can do. The Anzio is big and heavy, and probably takes two or three soldiers to operate efficiently; but it's not so massive that a lone soldier would be entirely unable to carry it around and use it if he had to.
I'd recommend setting maximum kinetic energy that a personal shield can block to somewhere between 30000-100000 J, depending on whether or not you want them to be able to block rounds from anti-materiel rifles like the Anzio.
With that frame challenge out of the way, let me move on to my actual answer:
Basically, take an antishield device (a cylinder 25mm in diameter and 70mm long), attach an arrowhead-like blade to the front and a small gunpowder charge to the back, and load it into a gun with a wide enough bore to accept it.
The flechette doesn't need to move very fast, since the antishield device is already capable of cutting through virtually anything. Thus, the flechette doesn't need a large powder charge, and because of that, the gun doesn't need to tolerate a particularly high chamber pressure. A construction similar to modern shotguns (but with a wider barrel) should suffice. If 25mm shotguns aren't already mass-produced in your setting, I'll wager that they will be soon.
But you're not asking what kind of gun would be preferred in your setting; you want to get rid of guns entirely.
To make this happen, you'll need to add a further restriction to your antishield technology.
Maybe the antishield generators are difficult to manufacture, such that using them as bullets would be prohibitively expensive. This won't stop snipers from using them to take out particularly important targets; but it'll at least force the common footsoldier to find another weapon.
Or, maybe antishield generators are fragile devices that would be completely destroyed if shot from a gun capable of launching it any reasonable distance. In this case, people might try using crossbows, which may (or may not- your choice) be able to accelerate the antishield device gently enough for it to survive.
Or, maybe the antishield generator requires a large amount of electricity to function. If your energy storage technology isn't up to snuff, it may not be possible to squeeze an antishield generator and a big enough battery into a package that can fit in a reasonably-sized gun.
In this case, I expect people will try unreasonably-sized guns. Or flechettes with a piezoelectric crystal between the blade and the antishield device, which would ideally convert the device's kinetic energy into electricity when it hits a shield, allowing it to penetrate.
If those don't work, the next best option will likely be the antishield harpoon: A spear with an antishield generator at the tip, launched by an explosive charge, and trailing an electric cable connected to a battery on the wielder's person.
If you somehow do manage to nullify all forms of ranged weaponry, then the weapons of choice on the battlefield will almost certainly be antishield polearms. Many different kinds of polearms were used in Medieval Europe, and I suspect that scifi versions of many of them will be tested, if not mass-produced. The traditional wooden shafts may be replaced with aluminum, titanium, carbon fiber, or something more exotic; the steel blades may be replaced with something lighter, since they don't need to hold an edge, because the antishield effect will be doing all the cutting; and of course each one will be outfitted with an antishield generator.
I don't know exactly what styles of polearms will see the most use; that depends on the details of how your antishield technology works. If the antishield effect requires a sharp edge to focus it, then blunt surfaces (like the "hammer" side of a poleaxe) will be useless. So they'll probably lean toward blades and spikes. Antishield halberds, perhaps.
Soldiers will also carry antishield swords and daggers, but only as sidearms. These will be backup weapons that the soldier can fall back on if their polearm breaks, or is damaged by an enemy's polearm, or runs out of power, or if they drop it, or if an enemy soldier manages to get past the polearm's blade, or if an enemy gets knocked down and needs to be killed quickly before they can get back up.
If you really want people to fight using swords specifically, then take the fights off the battlefield. Swords and knives are great self-defense weapons when handguns are unavailable. If you get mugged in a dark alley, you'll hope that you have some kind of weapon on you that can penetrate the mugger's shield.
Or, perhaps your culture uses judicial duels to resolve disputes, and requires them to be carried out using swords or other melee weapons of a certain length.
This all assumes that antishield weapons are resistant, if not entirely immune, to being cut by other antishield weapons. If antishield weapons can cut each other just as easily as they cut everything else, then I think spears will be preferred over other melee weapons, since it'll be more difficult for your enemy to cut a spear that you're thrusting at them with their own antishield blade than it would be for them to cut a sword or axe that you're slashing at them.
To defend against antishield spears, people might try "shields" comprised of several spinning antishield blades, like a handheld lawnmower blade, perhaps with a non-sharpened ring around the edge to reduce the likelihood of shredding yourself or your allies by accident. The idea is to chop incoming spears to bits before they have a chance to touch you.
Perhaps this device could be mounted onto the spears, with the spinning blades orbiting around the shaft of the spear somewhere near the tip. This would get the dangerous lawnmower blade even farther away from the wielder, further reducing the likelihood of accidents.