Nothing prevents you from uploading fullsize images to SM and then restricting size view/use from there, so no real need to "resize" on export. About the only time I use that function is when the receiver wants a specific edge length, or say Craigslist which only accepts a file "X" pixels on the long side.
Here's one to wrap your head around.... delete all your exported worked files from your HD once they are uploaded. You're keeping the source file in LR, so why keep the exported file too? You can always fire up LR and regenerate that file at a later date if needed, and you have it up on SM and accessible. Going that route saves space on your HD. In fact, why not push up a full-size 300dpi file and use SM as your backup? Keep your RAW file local (with a proper regular backup) and let SM be your repository.