Selecting a size is easy: just pick your snug-fitting t-shirt size. If you're plus sized, you may wish to size down for a shorter tail (the fabric that hangs down below the rings), but tail length is the only difference between sizes, so don't worry about safety if you pick "too big" or "too small". I offer seven sizes from XS to 3XL, because not everyone wears a size medium.
SBP slings are sized based on a snug-fitting t-shirt size, and should yield an approximately mid-thigh length tail in each size. This is fine for the vast majority of sling wearers. The thing about the length of the tail (that's the part of the sling that hangs down in front) is that it's really completely subjective. Some people prefer a very short tail; others like it knee-length or even longer.
New users often find a shorter tail less intimidating, while experienced users sometimes prefer a longer tail, but I recommend mid-thigh length because it's a happy medium for most users. A mid-thigh length tail gives you enough fabric to use the tail as a rain or sun cover, or to cover up when nursing, but not so much that you'll trip over or get tangled in it. It also allows you to be generous while you're learning to adjust the sling (a too-short tail means you don't have as much room to adjust), and leaves enough room for a pocket on the tail if you're looking for a little storage.
Ordering by t-shirt size will give you a sling that fits most people in that shirt size. If you are on the far ends of the bell curve (e.g. very tall and thin or very short and heavy), you may email, but it's likely your shirt size will still be fine and will probably still be what I recommend.
While I think it's nice to have a sling that's not "one size", especially if you're not the "average" size that those are made for, you also don't need to stress about choosing the perfect size. Sizing is not critical when wearing a ring sling; I have worn shirts from sizes L to 3X and could wear an XS sling at any of those sizes -- the tail was just really short when I was wearing a 3X. Ordering your t-shirt size is fine for the vast majority of users, and gets you a sling that fits better if you're not "average".
You also don't have to have one of the pre-determined sizes; if you would like something in-between, just make a note of it in the "special requests" section of the order form. However, please keep in mind that even wearing an extra layer under the sling will change the length of the tail, so unless you always wear just a t-shirt and your baby never grows, the tail length will vary somewhat as you wear it, and while I do the best I can to sew the slings to the stated length, there will always be a little variation in the finished product.
If your significant other is smaller than you, it's very likely you can both wear the smaller size unless there is a large disparity in your builds. As long as the tail is longer than 10-12", it is still afe; it can be tucked in around the rings or into the body of the sling if you don't like the look of a dangling tail.
If one partner is much larger than the other, unless the smaller partner wears an XS sling, the larger partner probably won't need a longer sling than the smaller one does. If one wears an XS and the other is a large or higher, you may want to consider two slings, or buying a slightly longer sling that the smaller partner may want to shorten while wearing (as above).
If there is not a large size disparity, a size between partners is fine, or if the larger partner would prefer a shorter tail, a size for the smaller partner is fine. It's really all up to personal preference, as the tail length doesn't determine how the sling fits after adjustment (as long as it's at least 10" long).