I have done a handful of buildings in N. I am working on one right now as a matter of fact, off of a drawing from a 1970s vintage Model Railroader.
What dimensions do you need? If you need prototypical dimensions, and you aren't modeling a specific building, I just use easy measurements... Like 1 1/2 x 3 or what have you. Selective compression and artistic liscence are a scratchbuilders friend.
If by measurements you mean scale conversions go here:
Scroll down a little and you will find the N scale excel and text files. This site is indispensable for scratchbuilding, and it has lots of other neato stuff. Yes, I said neato.
N scale breaks down like this: .00625" = 1 scale inch. a sheet of .010 styrene is just shy of 2 scale inches thick. 1/16" = 10 scale inches. Some people say 1/16"= 1 scale foot, but those people are WRONG. (it is close enough really) The difference is hundredths of an inch.
3/4"= 10 scale feet.
33 feet = 1 scale mile.
And a partridge in a pear tree.
Take your time, and it really isn't that tough to scratchbuild structures. N scale poses it's challenges, but if you can learn to scratchbuild in N scale, you can scratchbuild in any scale.
I started building trains for the money and the Groupies. Now I just do it for the Groupies.
If its your intention to scratchbuild in wood I would recommend that you purchase a " Chopper ". Trust me it makes life alot easier
As for the farm house in the real world they come in all shapes and sizes. Suggest you have a go at a cabin ( Shack ) say 15 x 10 with 9 feet high walls and the roof with a 1: 5 pitch a few windows and a door.