For shotgun shells. Do you want a higher or lower number for more knock down power . I have a box of number 5 and 6 shot. Seen 7 1/2 at store . I'm assuming that would be a sheet load ?

The lower the number, the larger the shot pellets are....thus, the larger the shot, the fewer pellets in each shell. Target loads are 7 1/2, 8 or 9 shot. Hunting loads will usually be loaded to a higher velocity than target loads.

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Shot size nomenclature is confusing --- in birdshot sizes, such as you mentioned, the larger the number, the smaller the diameter, so, #6 is smaller than #5, and so on, until #12, the smallest numbered size, followed by size "dust", which I've heard of, but never seen loaded in commercial shotshells. #12 is the tiny size used in .22 rimfire shotshells, for example. Buckshot sizes on the other hand range range from 000 ("triple aught") upward thru 00, 0, etc. to #4, in descending diameter. There are additional designations for non-lead shot sizes, as well. Not every increment of size is commonly available or even manufactured as far as I'm aware. You can find loaded shells in #6, 7-1/2, and 9, but I don't think I've ever seen #8 or#10 or smaller in standard guage shotshells, for example. This is tech trivia fraught with arcane measuring and descriptive systems --- approach with caution!

The short answer as was posted above is small number - bigger pellets - less pellets per shell.
Knock down with a shotgun is kind of a relative term. It's more like matching load to the intended target. Sticking with the 'bird' shot sizes you mentioned. Bigger birds like pheasant and grouse, maybe rabbit needs a larger shot size like 6. Dove sized targets need a lighter load like 8s, if you want more than feathers for the table. Turkey, needs #4 at least.
It's a matter of how many pellets in the pattern, and how much energy is needed to take down the game. Overall performance is also based on the weight of the shot charge, and the size of the powder charge.

Shot size number was based on how many pellets fit in a particular size circle - don't ask me what size circle, but that's what they used.

The circle is 40" in diameter.

And you can shoot a "sheet" with any size of shot. If you meant "skeet", 7.5 or 8 shot should work well. Skeet is a game using flat clay "birds".

T-Star

Not to hijack the thread but, what are "Drams" and how do they play into the load.

When did they change the way shells are marked also the old shells I have are marked like. 2 1/2-1-6 I know it is a "6" shot, I guess 2 1/2" long and the 1 is drams? This happens to be an old 16 gauge paper shell that sits on a shelf. I have boxes of others marked in the same format.

I have been curious about this for a while now and I never get the same answer twice.

I really like old shotgun shells

A "dram" is a unit of measure used in the Avoisdupois weight system of pounds, ounces, etc. It was used to measure black powder charges for shotgun shells. Therefore, the larger the drams, the more powerful the shell. With the transition to smokeless powder, the power (velocity) of a shotgun shell was expressed as "drams equivalent" since it was not practical to express the weight of the smokeless powder which is much less bulky than the old smokeless powder.

An example would be a 3 dram equivalent 12 gauge shell. It contains the amount of smokeless powder needed to give the velocity of a 3 dram charge of black powder.

The marking 2 1/2-1-6 is 2 1/2 dram equivalent-1 ounce-size 6 shot.
Dram equivalent is a reference/carry over from the old black powder loadings as far as velocity of the load.
Without looking it up in a book I believe this would be a load that travels @ 1200 fps.
So in the old days it would take 2 1/2 drams of black powder to get 1 ounce of shot moving @1200 fps.

2 1/2 inch shell, 1 ounce of #6 shot

The usual format is drams (equivalent), shot weight, shot size.

Your 2 1/2-1-6 would read as: 2 1/2 dram equivalent of powder, 1 ounce of number 6 shot. A field load suitable for rabbits and squirrels.

A more powerful load for ducks and pheasants would be 3-1 1/8-4: 3 drams equivalent of powder, 1 1/8 oz of number 4 shot. This would give a higher velocity with a heavier load of larger shot.