I am planning to replace all 19887 Westy fuel system hoses. Can anyone inform me about exact inside diameters of hoses:

1. Fuel tank - fuel filter
2. Fuel filter - fuel pump
3. Fuel pump - canister ( I have no idea what purpose it serves ) *
4. Canister - to fuel line **
5. Fuel line - thru firewall fitting
6. All other fuel system hoses in the engine compartment .

What material line # 4 , marked ** is made off? Plastic, metal? 
What purpose the canister mentioned under number "4" does serve? 

Help is very appreciated.

Ark.