Your cards should be at least 200 x 200 pixels, and no larger than 5MB.
For large cards, the minimum recommended image sizes for each platform are as follows:
Facebook: 492 x 276 pixels
Twitter: 506 x 254 pixels
LinkedIn: 520 x 272 pixels
Google+: 530 x 264 pixels
For larger images you should simply maintain the above aspect ratios.
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