When you resize banners, you can only resize them down, not up.
You can go from 500x500 to 250x250 or 125x125
You cannot resize a 728x90 into 250x250, for example.
Here are some online tools and services you can use to achieve what you want.
Resizing banners
https://viewst.com/how-to-resize-a-banner/
https://promo.com/tools/image-resizer/
https://biteable.com/tools/image-resizer/
Banner makers
https://www.kooldesignmaker.com/
Do It Yourself banner makers.
https://www.mybannermaker.com/
https://onlymega.com/
https://bannerfans.com/
https://banner.fotor.com/