The ideal flue size is typically determined by the exhaust outlet of the stove or appliance. It shouldn't to be smaller than the appliance exhaust hole.

The Building Regulations (Document J, page 30) require that the minimum flue size for stoves are as follows:

• Up to 20kW burning smokeless fuel - minimum 5" - 125mm diameter.
• Up to 30kW burning any fuel - minimum 6" - 150mm diameter.

So the regulations recommend a minimum flue size of 150mm for any stove (on which wood could be burnt) unless it is Defra Exempt.

OPEN FIRES including flame effect open gas fires:
Minimum 200mm diameter for fire openings up to 500mm x 500mm. Larger fires require a flue area equal to 14-16% of the total area of the fireplace opening. (Minimum flue diameter 180mm for gas effect fires).

Pothanger 

This all in one pot hanging cowl sits on top of your chimney pot and suspends the chimney liner in place. It has a wire mesh to keep out the birds and a rain lid to help prevent water and wind coming down the flue. It has a tapered lid so the water runs off, a high rise inside to stop water getting in and a nice tapered rim so it fits nicely and looks aesthetically pleasing. This product is suitable for use with multi fuel stoves.

Fixing plate with screw fit rain cap

When there is no more pot on top the chiney, you can install your chimney liner either from the top or bottom of the brick chimney stack. 

Once the chimney liner is all the way down the flue to the point of the thimble, the next step is to connect the top plate to the top of the chimney. The main function for a top plate is to seal the existing flue from rain, wind and creatures from entering the chimney. It also has a clamp located on the inside collar of the top plate to support the weight of the liner.  

Screw the cap onto the flue liner.