That slot on the left can be used for a short SSD (2230, 2242), and a long SSD (2280). As for size a 2TB M.2 NVMe SSD is compatible with the 7590. This forum thread should help.
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Don't want to burst your bubble but I think you are confused. The 7590 doesn't have dual m2 for SSD. Only one for SSD and the other is wireless card. However, you can upgrade yours to 2tb capacity, it will work.