This study aims to examine the effect of rib set-out repeat on the structural characteristics and mechanical properties of double-knitted fabrics after dry relaxation and four washing cycles. The focus is on the behavior of the fabrics in terms of dimensional stability and stretch properties. Half Milano rib and nine variants of double weft knitted fabrics differed by the number of inactive needles in the rib set-out repeat were produced from a 25 × 2tex × 2 cotton/flax yarn on a 10-gauge flat-bed knitting machine. It was found that the rib set-out repeat affects dimensional changes in both directions: an increase in the number of inactive needles increases shrinkage. The rib set-out repeat also significantly influences the structural characteristics. As the number of inactive needles in the rib set-out repeat increases, the double-knitted fabric becomes denser, thicker, and heavier. The analysis of stretchability indicates that the repeat didn`t affect full lengthwise deformation but affected the full widthwise deformation. Fabrics with an identical percentage of inactive needles (50%) exhibit minimal differences in properties. The study results are particularly significant for a manufacturer to enhance the quality of knitted fabrics by understanding their dimensional stability and stretch performance.