The rootstock is the lower portion of the tree that you don’t see.  Rootstock selection and pruning determine the size of a tree.  If you have limited space, choose a dwarf or semi-dwarf rootstock and keep it well pruned.  If you want a tree to hang a hammock on, or to use as windbreak or shade, go for a semi-standard or standard.  The diagram below shows the relative sizes of different rootstocks.