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You can use 1920 x 1080 but the first image will be cropped. You can use this as a template for sizing that is roughly close to the preview size (since I couldn't find this information in the help - https://ibb.co/pnCrFdP ). Unfortunately it means I put the first image twice, once cropped, then the 2nd image full size (without the crop). Its a limitation but it works. You can check my models to see how it looks if thats what you want or not.
Also I think they might have changed it again when doing the website update so double check its size, I havent had a chance to yet but its on my todo list. Edit: Yeah looks like that template is now not fitting since the update, great that means all my previews are now off. ill do a test and see if I can get you an updated one since it also affects me now, arg! Edit2: This template should fit the size correctly for the preview image: https://ibb.co/ncJYmCQ . Essentially how you use it is keep your image at 1920 x 1080, overlay this as multiply and keep your content within the black frame otherwise that part will be cropped out of the preview. It can also serve as the first image if you dont mind the white space on the sides and it looks good as it is. Once happy, turn off the black frame and save.
