how to calculate how much trunk space?

I would simply use a tape measure.  Break the space up into bite-sized chunks.  It becomes a matter of breaking up the space into zones, and each zone will have a cubic size.  A zone can go all the way from top to bottom, side to side or it can be just a little nook in one side.  But they will all add up to comprise the total airspace.

Rounded areas?  While seemingly difficult to measure, they are not if you have zoned off the area.  If a rounded side comprises part of a zone, find the center of that radius and measure at that point.  You don't need to know the widest point and the narrowest point, just the middle ground. 

Back seat tilt back into the trunk space?  That is a zone, from the top of the seat straight down.  Use the wedge calculator found here to find cu ft of the area.  Rectangle calculator on that page can come in handy as well.  Thickness in the calculators would of course be "0" because you are just multiplying W X L X H.  Or just use a calculator.

Always draw a diagram first, put in the measurements, and then calculate.  Your diagram will show the zones of the trunk space and things will be kept reasonably simple.

You're nuts.  That's a lot of work, those peanuts.  And if the wind catches them they're all over the yard.

Better to use rocks.  You can get a good workout at the same time, and when you're all finished you can use them in your landscaping. 

Yeah, that's a good one!! I almost choked on my water!

Hey, or you could get the mattress from a waterbed, stuff it in your trunk, then fill 'er up. Measure the water afterwards.