Which connector block should i use?

I am trying to install a new light fitting. There are 2 sets of wires coming from ceiling, as this is a loop circuit i am aware that one of the black wires is the live switch connection and should be connected to the live wire from the new light however i am unsure about what size of connector block to use, i have a 3 amp, 5 amp and 15 amp connector block?

MIke
April 2009