Auburn might fit your description at this time, but there are always developers who will request variances (sometimes multiple!) so that their projects will be more profitable. Most properties in Auburn are 3+ acres, but in the last couple of years, we're seeing more cluster-type zoning, with one large development at .75 acres per residence. There are also some older properties (on the Manchester border) are under 1/2 acre, but these are limited to that area.
Partially because they have recently voted to send high school students to nearby Pinkerton Academy (in Derry) for their high school, Auburn has become a very hot market. Previously tuitioned to Manchester HS, there are now families clamoring to get into Auburn so that their HS students will attend Pinkerton Academy. Nearby Chester and Hampstead also tuitions to Pinkerton, but I don't believe that eeither of these has seen the increase in development that Auburn has in the past couple of years.
Please define what you would like in a charming small town. When I think of charming towns, some sort of town center with an independently-owned coffee shop, bakery or sandwich/pizza joint is one of the important things on my list. A place where people meet. You might consider Candia, somewhat more rural and 'undiscovered' town, or perhaps Deerfield. Both of these would fit your 90-minutes to Boston (at least during non-commuter hours).
Check back well before your 8-10 year retirement tho--things could change an awful lot between now and then