A pair of Prusik Loops used with a footloop or long sling are all you need to ascend a fixed rope in an emergency. They have the advantage that you can abseil off them if you get stuck, and they are dirt cheap! A standard length would be made from 1.5m of cord which allows plenty of tail for a double fisherman's knot. However, it is worth making one of your two prussik loops a shorter one and then you can also use this safely as an Autobloc (attached to your harness leg loop to safeguard yourself during abseils). For the footloop, either use a long sling, or 5mm cord that will tie to the same length as a long sling (2.5m is about right). We feel 5mm cord is the best for Prusik loops as it is thin enough to really bite into narrow ropes, strong enough (over half a tonne static), and light enough and of low enough bulk to carry on your harness at all times. Having said that, many mountain guides recommend 6mm cord for prusiks as it slides more smoothly on 11mm ropes and allows a greater safety margin. There is no "right answer" to this. NB Prusik loops are not suitable for self-belaying.
We supply Prusik loop cords in Mammut or Beal climbing cord depending on availability. It can be a bit shiny when new so may slip (beware), but soon becomes furry and grippy with use.
A Standard loop is just the right length for ascending and autobloc use if affixed to your belay loop. A Short loop is a slightly shorter loop that is just long enough, but can also be used as an autobloc fastened to a harness leg loop to protect an abseil. If you prefer fixing the autobloc to your belay loop and having it above your abseil device, then use the standard loop. The danger with using a standard loop and fixing it to your leg loop is that it may get sucked into your belay device.
A Footloop is a length of cord that when knotted is the same length as a "Long" Sling (ie 120cm). The advantage is that it is cheaper and makes a handy piece of abseil tat for emergencies. There is no real point in making a footloop from 6mm - it just means extra weight and bulk.