Drive-in Loading

How much mulch fits in the back of my pickup truck?

We don't know.  All trucks are different. Average is 1.5 cubic yards.

How much dirt can I get in the back of my pickup truck?

Average is 0.5 cubic yards.  The problem here is not how much you can fit, but how much the dirt weighs.

Should I bring a tarp?

It would be smart.  Especially for mulch or dirt. It is up to you.  After you leave the property, your load is your responsibility. 

Can I get less than 1 cubic yard loaded?

Yes. It's just priced by the cubic yard. So, people get ½, ¼, ¾ yards loaded all the time.  Depends what you can carry.