Silver chart TA, page-128

depends on your goal.. consider who you intend to sell it to in the future and at what price
if you plan on selling it back to the mint, they will buy any size but they take a fair scalp off the spot price

If you plan on selling it at the retail price, then perhaps smaller denominations are better as they will be easier to find buyers.. especially if it goes to a few hundred $ an ounce over the rest of this decade

If you are holding some physical PMs as armageddon money, then defo the smaller unit the better, you aint going to buy bread and milk with a 5kg bar of silver.

Personally i have a mixture.. probably 3/4 in 1kg bars and a few 1kg coins that i picked up at about the price of bars
the rest i have bought second hand.. either predecimal silver (i like the small denominations, and have picked up at and even below spot) basically my upper limit is 10% over spot for high circulation coins
Ive also picked up plenty of mint collections .. and my upper limit for those are the same price as a kangaroo from the perth mint.. so i am basically not paying for the box or the collectability.. silver is silver.. a picture of darth vadar or bart simpson adds no value to a coin for me. but it has the potential for selling at a premium in the future.

ive gotten my best bargains paying spot price for 50% silver predecimal, kennedy half dollars and 1966 50c rounds. ive managed to get several kilos of silver in coin form that way cheaper than buying bars from the mint
keep an eye on gum tree or the likes.. 95% will be asking too much.. but you get the occasional bargains, maybe even garage sales if you could be bothered getting up eartly on the weekend.. thats a bridge to far for me
hope that helps