As far as size goes, I enjoy the rather extra large size of my frictionless marine propeller shaft bearing. Its dimensions are 4 x 1 3/8 inches. The interior is lined with rubber, but I still have a roomy inch of space on the inside.
Why my little finger and not another? I don't know it just seemed to work best that way for me. There are many ways and means to an end.
I think it's brass but maybe it's bronze. All I know is that it has worked well for me in my particular high tension tuning (GDGBDG) on my particular guitar, my home made Nobro (National & Dobro hybrid). The extra heavy weight seems to function well to increase sustain, and the length seems to help with reaching notes on my, twelve frets to the body, neck.
The one drawback is that the shiny surface finish does get dull or scuffed up after playing and when dull it seems to get a little noisy. Therefore, it's been necessary to keep it polished up nice and shiny in order for it function best.
It was just a freak find at a garage sale one day that cost me $.50 but if I ever had to replace it at a marine supply house it would cost $50.
Many have made mention of it's size when seeing me perform. My comment is always, "It's not the size of your slide it's the slide of your size."
Before getting involved with metal slides, I could always be found rummaging through the trash containers in the alley behind some bar or another seeking the right size and shape bottle neck from a discarded liquor or wine bottle. I think some folks thought I was back there trying to drain what might be left of the beverages. This method of seeking out an Excalibur of bottleneck slides was always thwarted by the glass slides eventually falling to the ground or floor and breaking, hence, my move to the unbreakable properties of metal sides.
A friend said he used a 13/16 inch deep throat spark plug socket. I tried one of these for a while but chrome finish began to wear off. (Lousy chrome plating! Don't they realize that there's more than car fixing purposes to consider when they're putting a chrome finish on these tools? What about my blues sliding needs?) When I went looking for a replacement I found the 15/16 inch deep throat was more comfortable, but it wasn't long before another inferior chrome finish was once again worn away. When I went looking for yet another replacement, and couldn't find one at the swap meet, I opted for the larger still 22mm deep throat socket. This move to the metric system seemed to add a certain distinctive Euro flair to my otherwise original Delta interpretations. I'm not sure that the more strict blues fundamentalists, who seem to crave imitation and not innovation, would have approved of my metric selection. It wasn't long afterward that I ran across the larger yet cut-less bearing, as they're called and the rest, as they say, is hick-story.