Set-UserPhoto size of picture?

When we upload larger picture, Exchange can automatically resize these photos for use in different products as needed. I haven't found the official recommendation for the particular size, but as mentioned in the below Technet article, it can can automatically resize in three different photo sizes and resolutions:

• 64 pixels by 64 pixels, the size used for the Active Directory thumbnailPhoto attribute. If you upload a photo to Exchange Server, Exchange will automatically create a 64 pixel by 64 pixel version of that photo and update the user's thumbnailPhoto attribute. Note, however, that the reverse is not true: if you manually update the thumbnailPhoto attribute in Active Directory the photo in the user's Exchange mailbox will not automatically be updated.

• 96 pixels by 96 pixels, for use in Microsoft Outlook 2013 Web App, Microsoft Outlook 2013, Skype for Business Web App, and Skype for Business.

• 648 pixels by 648 pixels for use in Skype for Business and Skype for Business Web App Skype for Business Web App.

Article- https://technet.microsoft.com/en-us/library/jj688150.aspx