soo....u want custom but you are buying a kit? kind of backwards if u ask me..but w/e
there are alot of different sizes you can use for fittings. usually they will come with a G1/4 thread and the other end is either a barb or compression fitting. the barb end usually comes in 1/4in, 3/8in, or 1/2. i dunno if they use standard or metric in AU but those are the sizes =)
tubing will have markings on them that will tell you the size of the tube. i believe my tubes are 5/8 OD (outer diameter) and 1/2 ID (inner diameter) if you have 1/2in barbed fittings then you want 1/2 ID tubing.