Long and short refers to the wheelbase of the frame, the distance between then front and rear axles, its one measurement that makes up a light of common dimesions taken from a frame and generally termed the Geometry. Wheelbase will make a difference to how it rides, but there are many other aspects of the geometry that will make the bike ride differently.
http://http://www.tartybikes.co.uk/media.php?group=guides&id=22
Have a look at that, gives you a good basis on the difference between frames.
How long have you been riding for and where abouts in Bristol are you? Ride Bristol every now and again, so keep checking the rides thread for a Bristol ride and pop along.