For Instagram and Meta, the pick is both one single image for the feed and one for stories.

For Stories:

Design Recommendations
Image File Type: JPG or PNG
Ratio: 9:16
Resolution: At least 1080 x 1080 pixels
Consider leaving roughly 14% (250 pixels) of the top and 20% (340 pixels) of the bottom of the image free from text, logos, or other key creative elements to avoid covering them with the profile icon or call-to-action.

Text Recommendations
Primary Text: 125 characters
Technical Requirements
Maximum File Size: 30MB
Minimum Width: 500 pixels
Aspect Ratio Tolerance: 1%

For the feed:

Design Recommendations
Image File Type: JPG or PNG
Ratio: 1:1
Resolution: At least 1080 x 1080 pixels

Text Recommendations
Primary Text: 125 characters
Headline: 40 characters
Maximum Number of Hashtags: 30
Technical Requirements
Maximum File Size: 30MB
Minimum Width: 500 pixels
Minimum Aspect Ratio: 400 x 500
Maximum Aspect Ratio: 191 x 100
Aspect Ratio Tolerance: 1%

My pick is LinkedIn Forms, obviously for lead generation campaigns.

Works in combination with Single Image ad format.

So, straight to the point, here you have the specs:

For the Single Image ad:

Design Recommendations
File Type: jpg, png, or gif
File Size: 5 MB
Image Ratio:
1.91:1 (horizontal, desktop and mobile)
1:1 (square, desktop and mobile)
1:1.91 (vertical, mobile only)

Text Recommendations
Ad name (optional): 255 characters
Headline: 70 characters
Introductory text: 150 characters
Description (LAN only): 70 characters. Only required if using LinkedIn Audience Network (LAN).

Technical Requirements
File Type: jpg, png, or gif
File Size: 5 MB

Horizontal / Landscape
**Horizontal images will deliver to both desktop and mobile
Minimum: 640 x 360 pixels
Maximum: 7680 x 4320 pixels
Recommended: 1.91:1 – 1200 x 628 pixels

Square
**LinkedIn recommends this aspect ratio for delivery across both desktop and mobile
Minimum: 360 x 360 pixels
Maximum: 4320 x 4320 pixels
Recommended: 1:1 – 1200 x 1200 pixels

Vertical
**LinkedIn recommends this aspect ratio for best performance on CTR on mobile (vertical images do not deliver to desktop)
Minimum: 360 x 640 pixels
Maximum: 2430 x 4320 pixels
Recommended:
1:1.91 – 628 x 1200 pixels
2:3 – 600 x 900 pixels
4:5 – 720 x 900 pixels

Image of screens showing Single Image Ads
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For the LinkedIn Form:

Text Recommendations
Form Name: 256 characters
Landing page URL: 2000 characters
Offer Headline: 60 characters
Offer Detail (optional): 160 characters
Privacy Policy: 2000 characters
Call-to-Action: 20 characters
Confirmation Message: 300 characters

Form Recommendations
Form Fields: 3-4 fields are recommended as best practice. 12 fields are the maximum

For Google Display Network, the pick is an all-time-classic such as:

300×300

Square images help boost performance by ensuring consistent ad format appearance with minimal white space.

Resolution: 72 dpi.

File types accepted: JPG, JPEG, GIF or PNG. If JPG, JPEG or GIF, image must be saved in RGB color code with an ICC profile attached to it.

Sizes are no bigger than 150 KB. Anything larger will be rejected by Google.