In the reports of studies on rats, you see dosage defined in terms of weight, e.g. 100 mg/kg.

Is this always done in human trials?

How often is body weight considered in the day to day prescribing of PD drugs?

Back in 2002 Zappia et al, reported on this effect [1]:

" ... a greater percentage of women showed LD peak-dose dyskinesias compared with men. Our findings suggest that lighter patients with Parkinson's disease probably receive a greater cumulative dosage of LD per kilogram of body weight during long-term treatment, because in clinical practice, LD is administered without any adjustment of the dose to body weight. This could explain gender differences for the development of LD-induced peak-dose dyskinesias observed during the course of the disease."

It is possible that for drugs targetted at the brain (e.g. carbidopa is used to allow more of the levodopa to get into the brain) that brain weight should also be considered.

References:

[1] Clin Neuropharmacol. 2002 Mar-Apr;25(2):79-82.
"Body weight influences pharmacokinetics of levodopa in Parkinson's disease."
Zappia M1, Crescibene L, Arabia G, Nicoletti G, Bagalà A, Bastone L, Caracciolo M, Bonavita S, Di Costanzo A, Scornaienchi M, Gambardella A, Quattrone A.
http://www.ncbi.nlm.nih.gov/pubmed/11981233

John