Yeah, so basically, the file size will depend on the source or where the file is. A 5 MB file on your memory card would possibly be a different “size” on your computer or somewhere else and might depend on the image itself too (colors, contrast, etc.).
The Save button will keep the setting you have selected last in the Save As… If you don’t have any storage space issue, you might want to keep the compression set at 1 and have the highest quality you can get. If space might be an issue, you can check what compression level will yield a file size that is comparable to what you are starting with. Unless you want to turn a photo into a billboard, I think the compression might not make that much difference (as long as you don’t go to a value of 25+).