Panties need to be large enough not to cut into soft flesh. A muffin-top or VPL (visible panty line) is not a good look!
Buy the size that fits and don't worry about the number on the tag. Many underwear manufacturers have not been changing their sizing inline with dress-size changes (whose numbers are getting smaller). So, buying the correct size in slips and shaping underwear in particular can come as quite a shock! If you don't like the tag size you can always cut off the tag!
For those times when you need some extra control shaping underwear is the way to go! There's a foundation garment available to make your waist, torso, hips, thighs and bottom look firmer and smoother. Pick the foundation wear you need to fix your problem.
To test the fit sit down, because your body will expand when seated. Make sure it is still comfortable when you're sitting down. If the legs ride up or curl up, or the top rolls down try a larger size. You don't want your shapewear giving you extra bulges!
And walk around to check that your foundation garment stays in place and doesn't ride up when you're walking. If it does it's either too tight or too loose. Try the next size down first. It should not move up as you walk. Some makes of half and full slips have a silicon strip at the bottom to prevent it moving.
If you're short waisted you can tuck high-waisted pants under your bra. You can also do this with a longline bra. Some makes of high-waisted shape wear also come with shoulder straps to prevent it rolling over at the top.
A longline bra is good if you tend to get a bulge under your bra band.