In ardour, there are only a few predefined sizes for the tracks (about 5, I think). I must admit (and this is perhaps personal taste) that I like more the alternative way of doing this, clicking and dragging the edge of the track to the desired size, like you can do in Audacity, Cubase and other programs. I know that it doesn´t have a great relevance, but here is my humble proposal. 
Cheers.