Generally, "EDM requirements" are based on the design in use. There are only a few limitations - a standard size based on screen size - default is 600px, or450px for mobile phones, optimised images again based on screen size and file size.

It is recommended that for any standard EDM to have a header image that is event-oriented and would match the overall EDM layout. For consistency of design, the header image is usually on the Invite, Confirm, Details, Surveys, Registrations and PDF copies.

The Header/hero image is best presented at 600px wide, ( it can be a gif/jpeg or png ) and 400px tall

Any mage should not be over 2meg.

A logo (min. 100px wide, can be an image - is but best supplied as jpeg or png) and can be added per brand.

Hex-codes should be supplied for the primary, secondary and text accent. These ill be used in RSVP call-to-action button and background theme. These can be added and applied by brand...
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We can accept a 'brand Kit" and upload that to Event Hub for future guidance.

For Mobile phones - the EDM is often smaller perhaps 500-450px - note this is where you might be showing/hiding "blocks" based on screen size.