Desktop screen of this test: Retina display (@2x) 1280 x 800 px

Google+ profile image (proportions 1:1)

Upload suggested dimensions: 480 x 480 px
Upload minimum dimensions: 320 x 320 px
Upload formats: jpg will be saved as jpg, png as png

Desktop visualization

Main (reduced by css): 170 x 170 px (natural dimensions: 480 x 480 px)
In post list: 40 x 40 px
In comments (reduced by css): 32 x 32 px (natural dimensions: 64 x 64 px)
In notification list: 48 x 48 px (natural dimensions: 100 x 100 px)

Google+ cover image (proportions 16:9)

Upload suggested dimensions: 1080 x 608 px
Upload minimum dimensions: 480 x 270 px
Upload maximum dimensions: 1120 x 1192 px

Desktop visualization

Main (reduced by css): 910 x 512 px (natural dimensions: 1820 x 1024 px)

Google+ post image

497 x 373 px

Google post image (1:1?)

Upload suggested dimensions:

Desktop visualization

232 x 272 px

Google Rich Snippet/Structured data image

148 x 98 px

Google AMP page logo

Upload suggested dimensions: 60x600px rectangle (height: 60px; width: auto; max-width: 600px)
Upload minimum dimensions: height: 60px (suggested) or width: 600px
Upload formats: jpg, png, gif (raster, not vector as .svg)
Text: 48px tall and centered vertically against the 60px image height; additional space should be added to pad the height to 60px

Google Adwords Logo (proportions 1:1)

1200x1200px

Google Adwords Horizontal Logo (proportions ?:?)

1200x300px

Google Adwords Image (proportions ?:?)

1200x628px

Google OAuth Logo (proportions 1:1)

Upload maximum dimension: 120x120px

Source: https://developers.google.com/search/docs/data-types/articles#logo-guidelines