It probably doesn't make sense to assign parameters to agents at the parameter file level -- that's because there is a one-to-one relationship with the parameter file and the top-level model. What you should do is something like:
1. Create an attribute at the top model context called "Bug Size" or perhaps "Default Bug Size" if you want bugs to be different sizes.
2. In an initialization rule for the bug assign that default value to the bug's size attribute
For an impossibly technical and meandering discussion of some of the deeper issues of assigning initial state for nested parameters, you might be interested in this:
Hi thanks for your reply - we will do it this way!
No problem! After I replied to you, I remembered a thought I had for what might actually be a cool/useful feature -- and perhaps be simpler than the other solutions discussed in the above referenced bug. We could consider supporting some kind of XPath like assignment of variables. This would be directly supported by EMF, so wouldn't be incredibly difficult to implement. Then you could do something like: