Can anyone give me backpack size advice for central and south america?

From Cancun to Venezuela you're my hero I always wanted to do that but never had the courague to. I have only been up to Panama City and thats it. I would recommend a light back pack because it is very very hot in that area and I would not recommend sleeping bags or any sleep material because odds are you will stay in doors to sleep. If you ever end up sleeping out doors sleep in the plazas since they are safer or near a police station. Good luck on your journey I envy you. Take the lightest backpack possible.