Some good answers, but I have to chime in on this one...
It depends on your era more than anything else, but you will not find many FM roads in Texas with a right-of-way narrower than 50 feet, equal to roughly 6.9 inches in HO scale. The right-of-way is that swath of land from fence to fence including the paving, ditches, signs, utilities, etc. I assume when you ask about "easement" the right-of-way is really what you're talking about (it's more of a legal distinction than anything else).
In the modern era, TxDOT requires a 4 foot
[0.55 in.] shoulder outside the driving lane. Many times it's weed covered and is often just base course asphalt, so it can appear to be narrower. Years ago the 4 foot shoulder wasn't required, but the base course asphalt always extends past the driving lane, so there's at least
some paved shoulder. Not only that, the roadbed is graded wider than the paving section to avoid having the roadway edges deteriorate over time.
You may have lanes as narrow as 10 feet
[1.38 in.], but recommended practice is 12 foot
[1.65 in.] lanes. Yellow striping in between opposing lanes of traffic is one foot
[0.14 in.] wide (space dedicated for a four inch
[0.05 in.] yellow stripe, a four inch space and a four inch yellow stripe regardless of the striping pattern), and the outer white stripe is four inches wide, so really your lanes are only 11 feet 4 inches
[1.56 in.] wide when you use a 12 foot section. With a 10 foot lane, the drivable area between stripes is only 9 feet 4 inches
[1.29 in.], and since modern highway trailers are 102 inches wide (8 feet 6 inches)
[1.17 in.], there's not much room for error, hence the recommended 12 foot width.
Things get tricky for the dimensions of passing lanes, turn lanes, etc. which are based on design speed, grade, visibility and other criteria. I usually let TxDOT give me the dimensions after I take a stab at it. I guess it's a case of me throwing them a bone on what I think the dimensions should be and they come to the rescue with the correct answer.
Again, another assumption, but since you live in Keller and I do projects for TxDOT in DFW and Houston, I'm going by the Texas standards, which may or may not be what you need. But, in any case, I must recommend the Texas Manual on Uniform Traffic Control Devices because it's a well illustrated and clear handbook for how things should be designed and the space required for various situations. And the fact you don't have to be an engineer to understand it helps, too. Check it out:
http://mutcd.fhwa.dot.gov/resources/state_info/texas/tx.htm