Been looking at this a bit myself lately, and after a little bit of google maps measuring most of the national tracks use an area of about 35 x 25m...except for rhr which is a massive 45 x 35m in total.
In terms grass tracks...the cobra club in Cardiff have a permanent grass track that they use weekly or fortnightly throughout the summer. Last year it cut up quite badly and their regional race had to be moved to a fresh patch of the park they use.
I'm a fan of grass but not sure of it's long term durability without some real ground keeping skills if your going to use it based on that experience...but have no knowledge of other grass track clubs to comment further.
Astro does give better all weather driving, and better durability so I guess there's a reason that the nationals have only 1 grass track at oswestry in the calendar, and that's always built on a new unused area of grass from what I gather.
Good luck with your track
