If an auto parts store sold me a wrong size freeze plugthat came out and ruined my motor, can they be held responsible?

You may be able to sue the store for negligence.  Negligence is based on the failure to exercise due care (that degree of care that in this case a reasonable auto parts store would have exercised under the same or similar circumstances to prevent foreseeable harm).  In order to prove negligence, you will need to prove breach of the duty of care, actual cause and proximate cause.

Breach of the duty of care means the failure to provide the correct size freeze plug.  Actual cause means but for the store providing the wrong size freeze club, would your car have been damaged? If the answer is no, which appears to be the case, you have established actual cause.  Proximate cause means were there any unforeseeable intervening events which would relieve the store of liability?  If the answer is no, you have established proximate cause.  You have now established negligence and your damages (the amount you are seeking in your lawsuit) would be the cost of repairs to your car.  You will need to mitigate (minimize) damages by selecting a repair shop whose charges are comparable to other repair shops in your area.  If you select the most expensive repair shop you can find, your damages will be reduced accordingly.

If the freeze plug itself was defective, you would have a products liability claim against both the store and manufacturer.  Products liability would include claims for negligence and strict liability against both the store that sold you the part and the manufacturer.  Strict liability imposes liability even if the manufacturer and store exercised due care.  Strict liability imposes liability even if the store could not have determined that the part was defective.   Damages would again be based on the cost of repairs to your vehicle.

If the part was NOT defective and was only the wrong size, you do NOT have a products liability claim.  Your claim would only be for negligence against the store that sold you the part as mentioned above.