rowan trees are lovely, cause you get red berries in winter for the birds, or you could have standard trees which dont grow big, like acers, salix, small cherry blossom is lovely, twisted hazel,so many really,if you put small trees in google you will get some and will tell you how big they grow,

hi there i have just bought a dwarf japanese twisted cherry tree for my front garden and its dwarf it doesn't grow beyond 6 foot tall its lovely in blossom now and in the winter when the leaves fall the branches look beautiful it looked beautiful with snow on the branches not so long ago up here in the frozen north ....
bobbing along to the library and looking at trees for small gardens is a good idea thats what i did good to see the pictures and plan what the trees will look like in the different seasons .