Not a definitive answer but here's a datapoint for you. My house came with a 87K BTUH furnace and a 40 K BTUH (very common burner size for 40 and 50 gal residentials) water heater. Each appliance had 4" type B double wall branch that connected into a common vertical 6" type B vent that went out the roof. Pretty good bet 6" will be what you need.
Depends on the combined BTUs of both AND the total height from the top of the furnace to the top of the chimney and where you live. Need to get that info looked at with a gas code book for your area. There are also special regulations as to how you join the 2 together if you are going to run them in one pipe and then into the chimney. Best way is to have 2 outlets in the chimney. Water heater MUST enter above the furnace. They also require B vent from the furnace to the chimney and it be within 6 feet of the chimney if it is a mid efficiency unit. Best to get the info from a gas fitter in your area who has a code book and knows the rules and any local requirements. If not done properly smoke/fumes/CO can spill into your house. If done improperly the next homeowner can sue you if something goes wrong so there are some SERIOUS liability issues.
"Cut it twice and it is still too short".