I think that there’s considerable variation between brands, but in
general a 26-inch waist (depending on LOTS of other factors, such
as where the waistband sits, your hip measurement, body
composition, thigh circumference, etc.) would fit best in a US
standard size 4 or 6P (size 6 petite). Since a lot of clothing
manufacturers, in women’s sizes, tend move by odd rather than
even-number measurements (in Level 99 jeans, for example, 23 = 0,
25 = 2, 27 = 4, 29 = 6/8, etc.) you might occasionally be able
to/have to drop down to a 2, especially if your hips are narrower,
I imagine.