Thanks folks for all the replies - I am making my own hurdles so can adjust the size within the confines of the small field shelter/shed I am using.
I only have room for four pens (two either side of a gangway) so I can make the gangway a bit bigger. I’d based the 6’x4’ size on the commercially available pens I’d seen and forgotten the section in TT’s book but on rereading it, I may have subliminally taken some of it on board as I had decided to use eye screws and bamboo to hold the hurdles together.
TT does say that 5’x3’ is slightly on the small side so if I go to 6’x3’ or 6’x3’6”, that will give the ewes plenty of room and plenty of room between them.
The hurdles I made last year were just 2”x 2” with bits of 80cm stock netting I had lying around. These were very cheap, seem to be high enough and worked. Not seeing anyone else doing this - is there any problem with this arrangement?