A few facts on Slivers and Splinters
So it's pretty apparent that the Lord Ruler, while having Shard-like power, did not hold the whole of Preservation. I don't know what you mean by, "releases," but, I'd say personally that holding a tenth to a fifth of a Shard's power is probably enough to leave the residue of a Sliver on a person.
This also makes it seem that you don't need to hold the majority of a power, although now it makes me think that a tenth to a fifth might be a bit too low. "Large" is such a relative term when we don't have any concept of how much power a Shard holds. I don't really know how you would even hold a fraction of a Shard's power, aside from at a pool. We don't really know if people who give up large Splinters can become Slivers.