Here is a little info on the size and also of the populations in the towns that you mentioned.
Avon - area size 23 sq miles pop. 16,809
Farmington - area size 28 sq miles pop. 24,585
Glastonbury - area size 51 sq miles pop. 33,002
A few other towns to consider:
Simsbury - area size 34 sq miles pop. 23,867
Granby - area size 41 sq miles pop. 11,131
These are 2006 figures.
All of the above towns with the exception of Glastonbury are west / northwest of Hartford and Glastonbury is east of Hartford.
The Valley Book is a great source for information for most of the towns.
The Valley Book 2007:: Avon, Farmington, Simsbury, Canton & Granby CT community listings, restaurant, business directory
Good luck with your decision making.
Duane