The ad serving is not of the same size as the one requested.
Step 1: Log into Ad Manager > Inventory > Ad Units
Step 2: Search for the ad unit/s and check if you have the correct size/s in the settings. Edit and make corrections if necessary.
Step 3: Cross-check the ad sizes configured for each ad unit versus the sizes in Ad Manager
Step 4: If you have an ON-page setup, audit the sizes defined in your source code. Make the corrections if necessary.
Step 5: If you have Size Mapping enabled, add the relevant Bidder-level size/s in your Header Bidding configuration:
Step 6: Audit the Placement ID’s or parameters configured for each bidder to make sure they match the Ad Unit sizes.
Step 7: Re-run PubGuru Ad Inspector after making changes and see if the error persists. If yes:
For some bidders, you can log into their user interface and check the settings.
If unable to do so, reach out to your representative to have them re-check.