I have a 5000 watt generator and wanted to know if I could simply install a double pole breaker not sure what size, and back feed my service panel for when the power goes out. Turn off the main first of course

You can ONLY do this if you install a main breaker interlock. This defeats the main breaker if the generator breaker is turned on, and vice-versa.

DO NOT simply do it the way you suggest.

This is what Stan is talking about:
Generator InterLock Kit

It can be done as suggested with the interlock kit. However, with a small generator like that you may be better off with a small transfer panel which you can plug a cord into. They are available in 6 to 12 circuit configurations.

Missouri, I like the Interlockkit for down here, the problem is they use maybe 6-12 circuits, No others, It allows all 120 volt circuits to be on throughout the house, tun light and or fan only in rooms you need and you have the convience of available 120 volt power Everwhere.
We Have Hurricans down here, and think it is a Most cost effective solution for the $150. Even though it is only a couple of plates of stainless, it is effective, efficient safe.
You do need a power inlet box to feed this extra breaker.

I installed my 5KW generator using the main interlock kit Stan recommended for the same reason, so I can use receptacles and lights throughout the house, as long as I don't exceed 5KW.

My generator is located in an outbuilding for noise isolation, so I also installed a 30A generator breaker near the generator, fed the house with No. 10 wire, and installed a 20 Amp Generator Main at the service.
(The 30 amp breaker protects serves as a disconnect located within 3 feet of the generator, and also protects the No. 10's going to the house from short circuits. The 20 amp main at the service protects the generator from overloading, it will also open before the 30 amp breaker at the generator does, so I don't have to walk all the way out to my shed in the dark when running on generator power.