Choose chimney liner if you have a chimney :-) Measure from the floor of the room where the stove will sit to the top of the chimney pot. This is more than you need but provides extra for any bends etc. Also add the thickness of the two floors. I use a figure of 0.5m per floor so in a standard two storey property this is an extra metre. A typical post war council house is 8-9 metres. The above will provide more liner than you need, however it is a good idea to have extra due to chimney bends. Choose 316 grade liner if burning only wood and you are a light user. Choose 904 if you burn coal/smokeless fuel, if you slumber the stove frequently, or for belt and braces quality.
Can your stove have a 5" liner (easier to get down many chimneys)? Yes if a) the stove has a 5" collar AND b) is DEFRA-approved also known as smoke exempt (SE) AND c) manufacturer does not state otherwise. If the stove has this icon on the stove's page then yes it can have a 5" liner as it follows the rules mentioned. Otherwise choose 6". Some people choose 6" anyway (even stoves with a 5" collar CAN use a 6" liner). Why? Because 6" liners are less likely to be blocked by soot if you neglect sweeping the chimney (unlikely unless you burn damp fuel or slumber long periods). The ease of fitting of 5" liners should not be ignored! Our advice - go for 5" if you can ;-)
You can choose your "Fitting Pack" (everything else you need) once you are on the liner page.