Sideways phone photo? Turn it the right way up, or mirror it, and download the corrected picture.
Rotations and flips are applied to the full-resolution original, not the preview. Turning 90 or 270 degrees swaps width and height, as expected. Mirroring is useful for front-camera selfies that saved backwards (readable text in the background is the giveaway); flipping upside down fixes scans fed into a scanner reversed.
Does rotating reduce quality? The pixels are rearranged, not re-drawn. When the result is saved as JPG there is a single re-encode at high quality, which is visually lossless for a one-time fix.
Why did the downloaded picture change size? A 90-degree turn swaps width and height by design. The pixel count is unchanged.