Apple tree

Last December I bought a really terrible-looking apple tree (elstar?) from a basket in front of an Asda for 5 pounds, reduced at the till to 3. To my great surprise it is alive and well in the pot I put it in, and so keen to grow that I would like to plant it in the garden. I have long lost the label, so I have no idea what kind of tree this might be - a dwarf tree? A patio tree? I'd like to know what sort of size I can expect it to grow to before I plant it. Is there any way of knowing or of finding out?