I have a mid 60's Niagra furnace. I need to replace the combination fan and limit control. It has an 11 inch insert. I need to know if the insert length matters. I found an 8 inch control on Amazon.

SOURCE: need the cubic inches for the return air duct

For proper minimum opening you need to do duct calculation. This is not a typical math equation for the typical homeowner. And unfortunately alot of contractors do not take the time to calculate this.
If you insist on doing this yourself...... I would recommend the largest opening that a common filter will fit. probably 20" by 25". Going to big on a return air will never hurt. But going to small will.
20" x 25" will work safely 95% of the time. But... If you are running airconditioning that is 5 tons in size, you will need the design a return air system that will feed from BOTH sides of the furnace.

If you are just sizing the trunk line you should use a ductulator to calculate the minimum size. A good rule of thumb would be to size the "return air" trunk 1 - 1 1/2 times the size of the supply trunk line.

SOURCE: How to divide 12 11/16 inches into 71 3/8 inches?

you will need 5 full planks = 68 1/4 inches and the last board will need to be 3 7/16 inches wide.

SOURCE: my goodman furnace is blinking led 4 times i

check the flame senser connection it tight to the board and that it is clean or try looking at the fault codes again they are online you could check the air fliters are clean

SOURCE: Cust complained about no heat. Arrived at

Blocked flue pipe?

SOURCE: blower fan running constant after new fan and

Here's my best shot.
Check the temperature settings for the fan switch.
The low OFF setting should be about 100 degrees.
The ON setting should be around 150 degrees and this is the setting that "makes" the ON contacts.
The HIGH LIMIT setting should be about 180 degrees.
If these are set, can you use a meter to see if the FAN ON contacts are closed?
Lastly, it appears that one hot line should go to one side of the blower and the other hot line should go to the bottom FAN terminal with the other side of the blower being fed by wiring from the top FAN terminal.
Good Luck. Gary