Welcome to Science Buddies! This sounds like a great project and
you have some really good questions. When doing a science
project, the best place to find answers is to look in the
scientific literature and then find a method that is suitable
for your research objective. It sounds like you have already
been reading some literature, but here is some additional
information that I found: Here is an article where the authors
used 5 x 40 cm pieces of plastic bags weighing 300 mg and
incubated them while shaking at 24 degrees Centigrade. The
pieces of plastic were weighed every 7 days to the closest
milligram for 49 days and the pieces were dried with a fan
before weighing. This looks like a good procedure if you have
access to a scientific balance with an accuracy of 1
milligram.http://plasticsoupfoundation.org/wp-con ... urtles.pdf
Here is a comprehensive review article that describes several
methods for testing degradation of plastic bags. This article
has lots of references in the bibliography that you might wa