The Okotoks Erratic in Alberta is touted as the largest glacial erratic (https://nsidc.org/cryosphere/glaciers/gallery/erratics.html). I observed a very large glacial erratic in the Columbia River Valley a bit below Grand Coulee Dam. Below is an approximate comparison of the two erratics via Google earth.
Basalt erratic west of Grand Coulee Dam
Based on the imagery, the Washington State erratic is bigger than the Alberta one. This erratic in the Columbia Valley is sitting in glacial lake sediments and the valley bedrock in the vicinity is granitic. I had noted these erratics previously (glacial-erratics-near-grand-coulee-dam), but had not recognized this particular one at the time.
As long as megablocks and rafts are excluded, the above Washington State erratic may be considered a competitor for largest erratic designation.
Glacial transported megablocks and rafts are huge sheets of material that get frozen to the base of glaciers and transported could well be considered much larger erratics (HERE). Megablocks Stalker (1973) and Stalker (1976) mapped huge sheets of rock that were transported by glacial ice in Alberta that are square kilometers in area. Stalker (1973) stated "The writer would recommend that the term "erratic" not be used, at present, for this type of bedrock mass".
A possible explanation for the huge erratics near Grand Coulee dam may be that they were part of a megablock that was adhered to the base of the ice but readily broke apart as the ice extended out over the blocked Columbia River forming Glacial Lake Columbia. Perhaps basalt flows would be susceptible to this mechanism - but this is getting ahead of my understanding of megablocks and rafts.