If you have to crop or create new images, for things such as cutting out parts of larger images for better staff and character portraits, then 230x345px is the recommended minimum, or at least 460x690px to support the XL cover.
Large images will be down-scaled to either 230px width for regular covers, or 460px width for XL covers. So it is fine to upload larger images.
Recommended Banner Dimensions
Banners should ideally be 1900x400px for media entries.
Uploading slightly smaller or larger images is acceptable, but up-scaling can lower the quality significantly.