It has previously been suggested that if you buy a hose, which is the wrong size, you could put the hose in boiling water to stretch it. This will not work. You cannot stretch our silicone hoses. Our polyester reinforced silicone will NOT stretch between temperatures of -40C and 180C so water from a kettle will have no effect. If fact the whole point of silicone as a raw material is that it does not shrink, expand or crack at these temperatures.
Hoses are sold by internal diameter not external diameter. You need to choose the hose with the internal diameter matching the outdoor diameter of the pipe you are wishing to connect to.
It is imperative that you ensure you do not buy a hose which is too small. Even a few millimetres could result in shearing of the inner liner which can mean pieces of silicone make their way into your system.
It is also worth noting that buying a how which is too big will result over clamping which will damage the hose and create bulging for water or air to escape.