The line between the trees must support your food and it also tries to pull the 2 trees together.  It gets into trig to figure out how much tension will be on your lines but most calculators will do the work for you.  Estimate the angle between the 2 anchor points and the line form the anchor point to the point on the rope where the food is hanging.  It will probably be between 30° and 45°.  The 30° will put the most stress on the tree to tree line so I will use it as an example. ( (Weight of pack)/ Sin(angle)/ number of lines).  Lets say your food weighs 1LB (I know that is light but it is easy to scale to your actual food pack) and the angle is 30°.  1lb/Sin(30°)= 1/(.5)/(2) = 1LB.  So the Tree to tree rope must have a working load of your food pack weight.  So where do you find working load for a particular rope when the only list the breaking strength.  In rigging classes I have taken they say breaking strength should be 7 times working strength.  So to lift your 1 Lb food pack the tree to tree rope should have a breaking strength of at least 7 LB.**I have also seen experienced riggers use a safety factor of 5 but your ropes will wear out faster( probably only raise and lower your food pack 1000 times).
For the rope that goes over the pulleys it depends on the number of pulleys and if you use a becket.  Count the number of ropes coming down to the food pack.  Each of these ropes will share the load of your food pack. If you have a single pulley whit a becket hanging from the tree to tree line and a single pulley at the food pack you will have 2 ropes coming to the food pack. Stress on rope = Weight of pack /number of lines = 1 LB/2 lines = ½ pack weight. This line should have the same working load to breaking strength as above.  So to lift your 1 LB food pack the pulley rope needs a breaking strength of 3.5LB.