That's what I was thinking. Often the main difference between one size and the next isn't the waist but the length. I was thinking that since the waist is elastic, the seamstress will only need to pick the length that is right and cut the elastic to the length needed. I've seen comments on this thread from people frustrated when they make something in a size they thought was right but found out it's not long enough. Some commercial patterns list finished length of garments but not all do. Also, since sizing can vary so much depending on brand and style, it sometimes feels like a guessing game to get something strictly based on the size.
I don't think I've shared my 'cheater' method for applique on tees yet.

I'm always worried I'll ruin a tee as I applique (it's happened a couple times). Also, I have problems with my wrists and found that it's hard for me to wrestle with moving the tee shirt around as I applique. I decided to try doing the applique on a separate piece of fabric (a lightweight woven cotton) then add fusible to the back and attach to the tee.
The tee I made for Mya's Big Give was done that way as was the Jasmine tee I did for an earlier Big Give. I assembled the applique pieces onto the base fabric then appliqued as I would normally except I don't applique the outer edges. Once I had the applique done, I ironed fusible to the back of the base fabric and cut it to the shape I wanted then fused it in place on the tee. For Jasmine, I added the gold pieces for the 'frame' and satin stitched around it. For Tink I fused the oval and did a looser satin stitch around the edge of the oval. Since I planned to cover the edge with ricrac, I mostly used the satin stitch to keep the edges neat rather than for a finished look. Here are photos of those tees.
Anyway, I thought I'd mention this as I've found it works really well for most applique designs to be added to purchased tees. If my applique doesn't work out, I haven't wasted the tee. Also, depending on how the applique is stitched to the tee, it's pretty easy to remove the applique and move it to a new tee - such as if the original tee is too small or stained. The tees below I did on base fabric as described above except I DID stitch around all edges of the applique. I then trimmed the base fabric leaving a narrow margin of it around the finished applique. I used a tight straight stitch to attach the applique following the outer line of the applique (I used thread that matched the base fabric). I wanted the edges of the base fabric to be able to fray a little so I cut the fusible so it went just past the edge of the applique stitching but left the base fabric margin mostly loose. However, the fusible could go all the way to the edge and the edge could be satin OR basic zigzag stitched to secure it in place.
Note: The applique are IS a bit stiffer than if it had been stitched directly onto the tee but really not that bad.
BTW - something for people who don't applique to consider is resell tees that are well appliqued but might have problems with the tee itself, i.e holes or stains. You can cut around the applique leaving a narrow edge of the old tee then do as I described above to fuse and stitch to a new tee. I've done that for a couple ladies who had tees their kids had outgrown but had worn very little - if at all.