I hope you're meaning something close to hypergiant sizes when you say supergiant. That would be awesome. Other than the 13 hand crafted hypergiants I don't think anyone has ever found one that wasn't the usual A or B type up to 500 solar radii. One of the most frustrating omissions made by FDev.
(Edit/added)
The largest procedurally generated found so far is Outopps YF-W d2-1, I think, found by Aura Lorell. The sad part is, it's in the usual 35-ish solar radii bracket as any other procedurally generated one. The limitations on the Galactic Forge when creating random things is also confirmed by this star, as it is the most massive red giant star on record.