Some Samsungs have had problems, but most of those were internally installed. I haven't heard of any problems with external SSDs, but then, not many folks use external SSDs, either. As for size, maybe a bit larger than your USED space on the internal, unless you know you plan to add a lot more to it. My clone backup from my 1TB internal SSD is to a 1TB external SSD. I do have archiving turned on on the external, as a backup to TM's archives, but I let CCC be aggressive in cleaning out what it needs so that archive probably doesn't go back very far. If you don't use archiving, and if you don't think you'll grow much on the internal, you can use any drive larger than your USED space.