First, it is always best for the backup drive's/partition's size to exceed "some" measurement of your internal drive. By that I mean how much space is being taken up on your internal drive. For example, my late 2012 Mac Mini has a 256 gig SSD inside, but I am only using about 78 gig of its total capacity. So, on my 2 external Samsung 840 Pro 512 gig SSDs, each of them has 3 partitions, with two of the partitions being dedicated for my SuperDuper! backups for both of my Macs. Each of those partitions is 100 gig in size, and thus can easily accommodate the backups of each of SSDs inside each of my Macs. However, I keep my Macs "lean, mean, and clean", and thus manage the space utilized very carefully. Also, I do not need prior versions of my Backups. My weekly backups are fine, with the new one just replacing the prior one. Like Time Machine, I can configure SuperDuper! to do incremental backups and/or keep prior versions. But then of course I would need a larger drive.
So, as you can see, the size of your external drive you want to use for backups depends on your needs, and how "well" you manage the space used on your Mac's internal drive. Of course you can choose to just buy a larger capacity drive than the size of your internal drive, but it's difficult to say how large it needs to be. Again, that depends on your needs.
Additionally, would you want to purchase a traditional Hard Disk Drive (HDD), or an SSD? If you are only going to use the external drive for backups, and thus not access anything on that drive and/or not use it for running programs, then an HDD should be fine. It is preferable to purchase an HDD that has a rotational speed of 7200 rpm (revolutions per minute). A 5400 rpm drive would be OK, but again depending on your needs, some operations will be slower than with a7200 rpm drive.
Secondly, your question "Is Time Machine good enough" again depends on what you want/need. Myself, I want a backup that is bootable, and also makes it easier whenever I need to do a recovery and/or install a new version of the Mac OS (or even an existing one). Thus, that is why I choose to use SuperDuper!. (Carbon Copy Cloner is a similar, excellent product). Time Machine works fine, and most folks who visit these forums use it.