Dimensions for a moveable ark?

I'm sure this has been asked a lot but here goes.

Our garden is a reasonable size but the nasty privet hedge that separates us from our neighbour is very gappy at the bottom so our girls will need a moveable run for when we're not actually out in the garden with them.  I am aiming for up to 4 Dutch bantams, we won't really have space for more than that if we want the roosting quarters and nestboxes to be a reasonable size.
So we'll be building the traditional ark-shaped run, I've got the small-gauge wire and we are getting some wood later today.  What size should we make it?  The garden is long and narrow (20' x 90'), what we laughingly call 'the lawn' is a 10' x 10' patch, the rest is not very good flowerbeds, a future vegetable patch and a bit of a herb garden, with a paved area and pond that we can't use for the hens.

Thanks.