480GB and 1TB. If you're unsure what you need then goto the Apple icon in the top left on your iMac, select "About this Mac" and then look at the storage tab. If you're only using 250GB then you're good with a 480GB SSD but if you're using anything above that, let's say 340GB then we advise you goto a 1TB SSD as that space can run out quickly.
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