Hi, i have no experience with iStock, but im almost sure that they just resize the Jpg preview to 1,235px (longest side). I dont know if they expect bigger files for review process, if not you dont need to spend time sending biggest files.
About image stoling, there is nothing to do, at least iStock have an invasive water mark but some files with white predominant background are at risk, dont care too much.
Actually SS have a new system and they sell image you send as preview of your vector as JPEG if buyer want raster version, dont need to upload in a separate way; and larges JPGS as preview are a good strategy now.
Regards