Is this real goose down?

No. The fill is goose down fiber, a synthetic or processed feather product made to mimic down. It feels similar but is heavier and does not loft as high as true down clusters.

What size is it actually?

The listed dimensions are 96 inches long by 88 inches wide. That length fits a California King, but the 88-inch width is closer to a queen, so it will not cover a king or California King bed edge to edge.

Can I use it in summer?

Yes. The medium fill weight is described as lightweight enough for summer and warm enough for winter. Very hot sleepers may still find the sateen shell warm.

How does it compare with a real down comforter?

It costs less and is easier to clean, but it is heavier, less breathable and will flatten sooner. Real down lofts higher and lasts longer for a higher outlay.

How should I care for it?

Wash on a gentle cycle with a mild detergent and tumble dry low with dryer balls to redistribute the fill. Avoid frequent washing, which speeds up flattening.