MyBB Community Forums

I need to know what could be acceptable  logo sizes and banner sizes if I want to change them?

You can choose any size you want.
Just create your own logo with your desired size and file format, save that one and upload to the imges folder.
Then go to ACP -> Templates&Temes. Select the used theme and scroll down to set this file as "Board Logo".
That's it

[ExiTuS]

Thanks for your answer. I do understand how to set the logo image from inside the Templates. There is a logo item there. But could you explain how I set a banner or header image? I don't find any items / boxes in Templates for a banner/ header.

A background banner is an individual modification not set in ACP.
This can be achieved by using CSS and the background properties, e.g. in global.css:

#header {
...
background: url(images/your_banner.png);
background-repeat: no-repeat;
background-size: cover;
}

"cover" will stretch an image to the entire element area. So better consider the image size of your banner to fit the header accurately.

[ExiTuS]

Thank you. Will follow that