Just to confirm by newsletter page are you referring to a marketing email that is sent out? the default email body width is 600px and the email body padding is 30px. I would recommend checking the default configuration settings of the marketing emails that are being sent, by following the steps in the knowledge base here
Once this information is got, there is a useful blog post here on resizing an image without losing the quality!
Hope this helps! If we were able to answer your query, kindly help the community by marking it as a solution. Thanks and Regard.
Background pattern of the Newsletter. Either too stretched or it repeats too many times
We need to see the code to understand and offer support for this type of question. If you have a link to "View the Email in a browser" that will suffice.
In short, this is likely due to the CSS code which is either stretching or repeating the image which would make sense based on your response. And because email clients are tough to deal with and support different things (see this reference - https://www.campaignmonitor.com/css/ ), what you are asking for may not be easy or in many cases possible to support in all clients.