The Dropbox API offers the ability to get thumbnails for images files via the /2/files/get_thumbnail and /2/files/get_thumbnail_batch endpoints. Note that it only supports retrieving thumbnails of certain pre-determined sizes though. Click on "ThumbnailSize" to see the list. The particular sizes you mentioned are not supported. If you need different sizes, you'd need to download the original files and resize them client-side.
Those are links to the documentation for the HTTPS endpoints themselves, but we recommend using one of the official SDKs if possible. Those have corresponding native methods for the HTTPS endpoints.
