Hi. Unfortunately Instagram only provide two image sizes to work with.
If you need more control I suggest manually sizing the images with CSS. You would probably need to select the large image size and scale it down.
Scott, sorry but that is no longer truth. I discovered that medium size is “low-resolution”. So I changed standard to low. If you could, please add it :). Thanks
Thanks for letting me know, it looks like Instagram have made some changes to the sizes they provide.
I have added support for this new image size (https://github.com/scottsweb/wp-instagram-widget/commit/2192da0d0ee31cc79b5a2dbf95b1a6f68284b4d4) and it will be released via ww.wp.xz.cn soon.