What were the biggest guns(naval and land) that saw action in the Napoleonic Wars? I think that muzzleloading operation set a limit for a gun's possible size, and made anything huger than a 12 pounder impractical. But what about experimental breech loading artillery that would be possibly quite big, even in the Napoleonic era technology, were they ever built? If so, where where they used(probably siege warfare and fort/anti-naval warfare would be most practical as it would be really hard to move such a big gun)?
In 1716, Colonel Albert Borgard, working for the Royal Ordnance, standardized British cannon. Cannon were rated by the weight of the shot they fired. The cannon sizes were 4, 6, 9, 12, 18, 24, 32, and 42 pounder guns. These standards lasted through the Napoleonic Wars.
http://www.arc.id.au/Cannonballs.html
Naval Cannon:
Ships of the Line would often mount 32 pounders on the lower gun deck, HMS Victory, for example, mounted 30 32 pounders, and 28 24 pounders, whereas the great French battleship L’Orient, which blew up at the Battle of the Nile, mounted 32 36 pounders on her lower gun deck and 34 24 pounders on her middle gun deck– the French used slightly different standards than the British). Frigates would often mount 18 Pdrs. (U.S. Frigates in the War of 1812, such as the USS Constitution, mounted 24 Pdrs, but they were powerful frigates). Smaller ships would mount smaller cannon. The long 9 Pdr, especially in brass or bronze, (they were more than twice as expensive) rather than iron, was often mounted as bow or stern chasers, as they were deemed to be especially accurate long range guns.
Carronades were short barreled guns, lighter in overall weight than equivalent long guns. As such, they could be mounted on upper decks, and throw a heavy weight of shot (either round shot or grape shot), but they were shorter range and less accurate than long guns. Carronades were first developed in the 1770s.
Ships of the line often mounted 42 Pdr carronades on their upper decks. Heavy frigates sometimes mounted 32 Pdr carronades. There were a few 68 pound carronades. HMS Victory mounted two of these.
http://www.globalsecurity.org/military/systems/ship/sail-armament.htm
Field Guns:
The heaviest field guns used for land campaigns in the Napoleonic wars were 12 pounders. In the late 1760s, Jean Baptiste Grimbeauval redesigned and standardized French Army guns. His designs were much lighter than previous cannon, and this allowed Napoleons armies to use 12 pounders as their main field armament.
http://www.wtj.com/articles/napart/
Howitzers and mortars:
The French employed 8 inch, 10 inch and 12 inch mortars (and other armies used roughly the same sizes). These were employed in sieges or siege defense.
The French also employed an 8 inch howitzer (firing exploding shells like a mortar, but over lower trajectories and longer ranges).
Coastal Defense Artillery.
Coastal forts usually employed naval cannon as their main battery in Napoleonic times. The 42 pounder cannon, which was not used at sea, was sometimes used in coastal defense forts. The US introduced the 50 pounder, 1811 model Columbiad as part of the “Second System of US fortifications” in 1811.
https://en.wikipedia.org/wiki/Coastal_artillery
That covers the largest standard artillery used in the Napoleonic Wars. There may have been a few exceptions, or experimental guns which were larger.
I can't say if it was the largest, but some of the Ottoman guns of period, many of which were antiquated but remained in active service, were bloody enormous: https://en.wikipedia.org/wiki/Dardanelles_Gun