Which formats and sizes are supported?
JPG, PNG, WebP, HEIC/HEIF, AVIF, GIF, BMP, SVG, ICO and TIFF, up to 20 images, 20 MB each and 100 MB per batch. Images and outputs are limited to 40 megapixels and 16,384 pixels per side. Exports are JPG, PNG or WebP: with Save as on Original, a format the browser cannot write is saved as JPG, or PNG when the picture has transparency. Animated files keep their first frame only.
How do resize modes work?
Without Resize, the export is exactly what is inside the frame. Add Resize and choose Pixels: it starts from the cropped size, and with Keep the frame's proportions on, typing one side calculates the other. Turn that off to choose how a different shape is handled: Fit preserves proportions within the bounds, Fill crops to the exact shape, Pad adds a background, and Stretch changes proportions. Percent scales the crop. Enlargement is off by default.