Blue Tubes/Pipes

20 or 25mm should be fine for most beds. probably upto about 8foot wide ( i maybe wrong)
but if its free, then it really doesnt matter 

OK, thanks Mum.

I use the white water piping, must be 20mm, just wide enough to put a piece of cane, or old tubing from a garden arch, in the ends to stick into the ground. It is a bit bendy and doesn't look as smart as the thicker pipe, but works all the same.

This is what you want

http://www.wickes.co.uk/invt/420600