Does Grimes County Texas have property size requirements to install a septic system? ?

Go to the Grimes country website and look up the county requirements for building sites.  In it will be the requirements.  Mostly pertain to RV sites, etc.  Also call the county ag agent, the county precinct office, etc and ask them.  Finally you will find the design rules on how to build a septic system in many books and articles, as well as septic tank installers. You will find a consensus design that you will have to decide that is what you want.   Finally, I will tell you how.  Buy the largest 3 compartment (about 1200 (?) gallons) concrete septic tank they make.  Put it near the house where no traffic gets on the lid.  Then run a sewer line out of the tank about 100 feet and slightly downhill  Then branch the sewer line into 4 drain lines, each about 80 to 100 feet long and tied together with pipe on each end.  In each of those drain lines put in about 18" of septic line gravel (1-2 inches and about 30 cubic yards (?)) and cover the gravel with something like weed block to keep the dirt out of the gravel.  The thing is you can never go too big on a septic system.  You will just never know of the problems you didn't have.  If you undersize, you will have problems.  Pump the tank out after about 6 or 7 years and see how much sludge you make.  From that, make a guess as to when you need to do it again.  It can be more than 10 years.

Thats a better question for Grimes County code department. 

ask at the building review department -- the people who issue building permits -- you'll almost certainly need one