Not every garden is estate size. Small gardens are becoming more popular. Small sized gardens to do not mean that you have to forego having trees. Trees that have maximum heights of 20' are most suitable. Since there may only be space for 1 tree, you need to make a good selection as this one tree will probably become the focal point of the garden. You also want to consider a tree that is attractive in more than just one season.
A Japanese crabapple (Malus floribunda) produces masses of crimson buds, then pale pink or white flowers and finally small red or yellow fruits on arching branches. This is just one example of things that could be considered for the small space garden.
Since almost half the year the deciduous tree will be leafless, it's important that the tree have an appealing appearance during this time, especially if the garden is visible from indoors.
A couple of other good examples include the staghorn sumac (Rhus typhina) and Pendula (Pyrus salicifolia). While neither of these two trees have showy flowers, they do have striking foliage and a good winter silhouette with lots of character.
Prickly trees such as hawthorns or hollies are probably not the most ideal since they may restrict movement in and around them. Trees such as lindens that drip sticky sap would also not be good candidates. If you plan having plants growing underneath, then deep roots trees would be preferred over shallow rooted trees such as some ornamental cherries.