Unless you select something known to be a dwarf cultivar......and with a growing history that substantiates that.......there are very few trees that will stop growing at 6-8 feet. Japanese maples are one of the few that come to mind and very few of those will stop at that height requirement either. And pruning trees to maintain a short, specific height when they want to grow much taller is not recommended both for aesthetics as well as the health of the tree.
Instead, you should be looking at a shrub that can be trained as a tree.....a panicle hydrangea, ninebark, rose of Sharon, dappled willows, dwarf Korean lilac, Prunus incisa, etc. are all possibilities that can be purchased already in a tree form or are relatively easily trained in that manner.
Q