There is no correlation between weight and dosage with finasteride. The type of drugs where this is an issue, are usually those with much greater liver affection (paracetamole i.e) or toxins (methanole), where more body tissue (weight) can handle lager doses.
The tried and tested dose of finasteride for hair loss is 1mg/day.
Results have been achieved with as little as 0.2 mg/day, but the best results was with 1mg.
There was little or no difference in results between 1mg and 5mg, and the side effects where basically the same with 1mg as with 5mg.
The only reasons I can think of to take other doses than 1 mg is..
*1.25mg- splitting proscars in quarters because it's easier than splitting it in fifths, gives the same results, and the extra 0.25mg will probably dissipate during cutting anyway.
*2.5- long time users (10 years ++) who feel that finasteride looses it's effect increases the dose to half a proscar. Some say it helps, but it's not proven.
*5mg- if you have benign prostate hyperplasia.
*0.25 or 0.5- if you're REALLY paranoid about side effects (not proven that smaller doses give less side effects, but some say it helps), or for economical reasons (splitting the expensive 1mg propecia into halves or quarters)