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When passing --size=auto to repart, it will pre-calculate the image size and resize the image to that size before partitioning. Currently, that fails when passing a large grain size, complaining that the auto-sized image is too small to fit the data. The reason for this is that the current code simply assumes the GPT metadata size taken away from the usable size by fdisk is static (1044KiB), when it actually is more complicated than that: There's two ranges of GPT metadata: One at the beginning of the image, and one at the end of the image. And there's the first usable block that is defined by fdisk when creating the partition table. The static value of 1044KiB usually works, because fdisk sets the first usable block to 1MiB (so 1024KiB), leaving 20KiB of leeway for the secondary GPT at the end of the image. Now as soon as the first partition starts at an offset higher than 1024KiB, we lose the 20KiB leeway for the secondary GPT, and the partitions will no longer fit. What we should do, is first of all round up to the grain size instead of 4096 (as that's the minimum offset our first partition will start at), and second of all properly subtract the secondary GPT at the end. Also confirm we don't regress on this anymore by adding a test that uses a 2MiB grain size, breaking the old code.