You have a choice of one colour – black, and a choice of sizes 20mm XL, 22mm black buckle, and 24mm mat buckle (not all straps are available in all variations).
This refers to the width of the gap between the two stanchions which house the strap. If you are unsure, the size is stamped on the inside of the watch strap. Please note that at the point of attachment to the watch, if your existing strap follows precisely the circumference of the watch and there is not a gap between the two, then you have a “Retro” model watch. If yu are unsure of which strap you need, this page may be of help
The total length of the XL strap is 92mm from the securing point to the buckle pin position, plus 103mm from the securing point to the last hole; total 195mm. Please note that the XL version differs from the others in that it has a reinforcement strap along the length of the strap and the tail is tapered. The extra long version.
Fitting the strap is easy, At this page there is a video detailing how to take off the old strap and replace it with your new one.