As a consumer for many years, I became used to soap sizes of the most common brands of soap one buys in the Supermarket and I am guessing that is what the majority of people have become accustomed to as well.
However, one day I bought or was given a bar of Rose-scented soap that was about twice the size of normal store bought soaps. I used that large size soap in the upstairs bathroom only, so not very often, but I came to really like the larger soap bar much better. None of the regular commercial soaps come in that larger size, so until I discovered that I could make soap any size I chose I was at the mercy of the commercial manufacturers.
Hand soap, however, I prefer to be what we call guest sized soap, or what I have always called hotel sized soap.
Now that I make my own soap, I make a variety of sizes depending on a variety of factors, the purppose, who it is for, the mold, my whim, etc. My granddaughter's facial bar is quite small, less than 2 ounces in weight, because at first it was experimental, and because the mold is dainty and girlish. She seems to like that size for her facial bar.
When I use a slab mold, which for me is still a cardboard boot box, I cut the soaps to a larger size. But like you I haven't really concerned myself with exact measurements so much as to attempt uniformity within the bars coming from each specific mold.
I make small bars and I make large bars. For my brother's and my husband I like giving them the large bars. For my DIL & SILs I like giving them more dainty looking bars, and although that is not always small, it often is on the smaller side.
Perhaps that gives you no specific guidance as you were seeking, but I believe it just demonstrates that customers are already used to a certain size and that tastes also vary, as well as the purpose of the soap contributes to desired size.
