I was also worried that our garden would be too small for the NGS, but was very pleased when we were accepted.
The last three years of opening for the NGS have proved to be wonderful, you get to have wonderful compliments from visitors and make money for a very worthy charity.
Visitors tend to linger interested in our plant combinations and garden design. That combined with the teas means that most visitors easily find 45 mins of interest.
It obviously helps if you have a particular "hook" for your garden. For us, it is our dogs, but it could just as easily be a particular plant group, colour, soil type, art or some such thing.
http://www.barhi.net/garden
http://gardeninganddogs.wordpress.com