Re:
I thought they did 16, 18, and 20 in all years... Not sure whether all sizes were imported to the UK in all years. Raidan73 must be right, as I can't recall seeing a blue 95 in the larger 20 size.
Easy to tell the difference:
Size 20 tends to look like a gate (chainstay / toptube angle);
18 is the most common and looks nicely balanced;
on a 16, the kicker tube joins the downtube quite low, which shifts the KONA decal down the downtube until its lower end is almost obscured by the chainring... they look mental in that size (in a good way).