HubSpot CMS does not require a specific image size for featured images, but your best bet is to follow recommendations from popular social media platforms like LinkedIn. If you upload an image that is at least 1,000 pixels wide you should be fine.
1200 x 630 pixels
1.91:1 aspect ratio
Minimum image width of 1,000 pixels wide
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The above size is for pages but what about HubSpot email banner sizes?
Recommended HubSpot email header size
For most templates, they should be a maximum of 600px wide and any height.