"............ Can someone please tell me what the volume of water is, in
gallons, of a 50 metre length of garden hose.

Thinking of making a heat exchanger to heat up the water in a big box with
50 metres of garden hose snaked out and under a Perspex cover ........"

I would have thought you would have used a header tank and a circulating
pump, in which case the volume is not important.
Have you thought of using black alkathene pipe, whilst it is a bit more
expensive you can get a better range of sizes and being black it will
absorbed more heat.
You say under a Perspex cover, if you can insulate under the pipe and then
have a reflective surface on top of the insulation then for the top use
something like Correx (Double skin sheet ) you will get more heat in the
unit.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk