24" circular needles are most commonly used for circular sweater knitting.
I recommend this length.
However, you could cram the stitches for a sweater onto 20" circs. 20" circs are not a common length. 16" are common.
What brand are you considering that offers 20"??
I've never heard of a 20" length in the mainstream circulars.
Circs length is measured from tip to tip, not just the cable.
Knitting on 16" circs 'in the round' can be quite difficult with the wrong brand. KnitPicks are the best for 16" length. It has to do with the pliability of the cable, AND, the length of the needle tip itself. KnitPicks 16" circs are just 3-3/8" tips. Kinda stubby looking, but they allow you to make the circle easily.
Here is what they say:
"The 16" needle tips are 3 3/8 inches long, allowing for an easily formed complete circle for small circumference knitting. The 24", 32", 40" and 47" needle tips are 5 1/8 inches long. "
Here is a photo of the fixed circular 16" needles"