Q: i dont know the size of the heater core is it 5/8 or a 2 inch

The 5/8 inch dimension versus the 2 inch dimension simply refers to the "thickness" of the core body. If you have your core out, or can wait to access it, just simply look and see if the core is very thin (5/8 inch) or really thick (2 inches). The 5/8 inch core is listed in the catalog for both the Durango and the Dakota, which set of applications matches the OEM catalog so my guess is you have the 5/8 inch core but again if you have to remove the core anyway, you will be able to confirm the dimension once it is out. Based on product reviews on this core as an aftermarket product I do NOT recommend you buy the core as aftermarket product. The only guarantee you have that it will fit (and not, seriously, have to be modified or forced into place) is to buy the core OEM from a dealer (including on-line) source. Furthermore, if you buy it from the dealer, there will be no uncertainty at all regarding core thickness. You simply give the dealer your VIN number, they hand you your part, no confusion and again, most important of all the part will actually fit and not cause you endless installation headaches. YourMechanic would be pleased to procure and install this part for you. If you desire a prompt and professional installation of the heater core by a certified mechanic, please simply request heat diagnostic/heater core replacement.