TH,any rat 5 or 6pounds is a huge rat in anyones book.I have never weighed a muskrat,but would doubt that a large rat here would be much more than 3 pounds.
When measuring a rat for grading,I measure dried ones only.They are measured from the nose to the shortest point on the side skirts.A rat of 13 1/2 inches ,is a large.The largest we have on the stretchers now is probably just over 17 inches.I believe anything of 16 1/2 inches is a xxlarge.Now I would have to check with the nafa charts to be exact on these sizes.
As far as aging goes,you are on your own.The lifespan of a rat is similar to a rabiits---short.Some of the rats born in the spring,will already have their first litter by fall.A rat lives a short and fast life,But I guess that is to be expected of anything that is everyones #1 meal.
Tom