Please read documentation:
https://winscp.net/eng/docs/resume

When using internet storage, the available space is often limited. Uploading large files is often a lengthy procedure and because of that, more subject to net outage then an upload of smaller files.
To be able to resume a transfer, one must use temporary files. This requires double the space on the storage server.

I would like a feature the allows for resume of the transfer without use of a temporary file of the same size.

I realize that this make the file unusable until it is fully uploaded and that it is not suitable during replace uploads.

Couldn't a small control file do the job?