Have a 2010 Fiesta After with 16 inch upgrade alloys fitted with 195\45\16 tyres at the factory, when we bought the car it had the can of gunk and the compressor so we bought a space saver spare which is 15 inch but now doing 200 mile trips more often I am looking at getting a full size spare. It will be a 15 inch steel rim but what size tyre would I need to get the same rolling diameter as my 16 inch wheel also would it fit in the wheel well in the boot.

Just use this website to calculate the difference. Obviously depending on the width of the steel rim would depend on what profile you chose. 

You cant give a definite profile size as it the profile is a percentage against the width of the tyre rather than a set figure in mm for example. Therefore if you have 2 wheels, one fitted with 250/55/r16 and another 150/55/16 then the latter will have a much smaller circumference.