The German UDC has measurements for the various 661 armour sizes: 661 4x4 Armrour
The important measurement there is “E”, the distance from your knee bend to the tops of your shoes. If the armour is too short in that dimension the armour will slide down your leg and expose your knee. The other dimensions will generally fit you unless you have really skinny legs or really rotund legs. The straps will cinch up or stretch to fit you.