For your 54 litre tank, the smallest one would be fine. They just vary in the with of the plastic end piece. I have discovered that the tubing is the same length whichever size you buy.
The way to start the siphon depends on the brand. With some there is a plastic bulb that you squeeze to start the suction. Some have a one way valve where the plastic bit joins the tubing and with those you shake the plastic bit up and down to start it siphoning (I recommend practising with a bucket of water with this type) And there are those that you have to suck the bucket end of the tubing to start it.
All fish shops will sell siphons of one type or another.