how can download fullsized photo from icloud shared albums?

shared Photo Stream holds photos up to 2048 pixels - so the full sized original may not be there

iCloud Photo Sharing supports JPEG, TIFF, PNG, and most RAW photo formats. When shared, photos taken with standard point-and-shoot cameras, SLR cameras, or iOS devices will have up to 2048 pixels on the long edge. Panoramic photos can be up to 5400 pixels wide.

iCloud Photo Sharing FAQ - Apple Support

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If they are longer than 2048 Pixels

When shared, photos taken with standard point-and-shoot cameras, SLR cameras, or iOS devices will have up to 2048 pixels on the long edge.

You can share larger photos using Mac mail drop

You can use Mail Drop to send files that exceed the maximum size allowed by your email account provider. Mail Drop uploads large attachments to iCloud, where they’re encrypted and stored for up to 30 days.

• If you have an iCloud account and you’re signed in to iCloud when you click Send, Mail automatically sends the attachments using Mail Drop. Mail Drop attachments don’t count against your iCloud storage.
• If you don’t have an iCloud account, or if you’re not signed in, Mail asks you whether to use Mail Drop (to always use Mail Drop select “Don’t ask again for this account”).

If a recipient uses Mail in OS X Yosemite or later, the attachments are included in your message. For other recipients, your message includes links for downloading the attachments, and their expiration date.

You can turn Mail Drop on or off for an account. Choose Mail > Preferences, click Accounts, select the account, click Advanced, then select or deselect “Send large attachments with Mail Drop

DropBox, email etc

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