I can break it up into chips/chunks, or use a planer to create long, thin slivers, or saw it into pencil size bars.
Instead of shape, think in terms of surface area. Dust, shredded, and chips are used most often for fermentation oak as the finer pieces have more surface area to interact with the wine during the short fermentation period. Chips, cubes, staves, spirals, and
Mike's product have less surface area, and require more time to impart oak flavoring to the wine.
Regarding duration, read through my
oak stix experiment. I provide some background on the common oak adjuncts, plus my tasting notes may provide you with an understanding how wine changing while aging on oak. The taste testing was a real eye-opener for me and my son. We got results, month-by-month, that we did not expect.
What shape(s) to use is a trade-off of simplicity vs. ease of use. Staves, spirals, and Mike's product are each to get into the wine, and easy to get out. Cubes and chips go in easy, but you need some type of container else you'll have to rack the wine to get them out.
If it were me? For aging oak, I'd go with pencil-sized rectangles as they'll be easier to use AND you have less waste wood. I'd use all the dust, chips, and small pieces for fermentation. If you need more fermentation oak, run a plank repeatedly through a table or bandsaw to make dust and small strips, or use a planer.