The videos show you tips on how to get it square when you don't have a squre table and a long enough Tbar. As we have a really big table we have always been able to lay out the width of any blinds we have made.
Sometimes in the workshop we have to butt an extra table up alongside when we are working on really big curtains to get the table space. Otherwise there is always the floor, infact at the moment we are working on a pair of apex curtains so big we had to take the tables out of the workroom and lay the curtains on the floor to get the angles on the heading right.
On the "Your Makes" page there is a 2.6m x 2.5m purple blind made by Heather, there is a little bit of info there on how she made it on a narrow table. (but I think it was long enough to take the width), the table is in the photos as well.