If it will be raining for an hour and then goes away for the evening, and you can be sure that you aren't likely to experience extended rain, I would lean towards the smaller side. You could just spend that hour in the hammock and go on with life afterwards.
As far as DIY goes, on a tarp it is quite the endevour for a beginner. I would just warn you that you could save yourself a bunch of time by looking into purchasing a tarp from a manufacturer. However, you can some money and feel awesome knowing you made your own tarp. I think I would even suggest making a prototype before you do the real thing if you are inexperienced. This is going to take even more time, and that is why I suggest comparing the time spent/hair pulled to going to work and making money to spend on the tarp. My first tarp almost caused me to quit sewing altogether, and taught me new curse word combos, but I am a better man for it!