A little confused here... are you talking about 5-6' tall 'bonsai' or just trees in garden pots, or ?? Have you done bonsai before? Dawn redwood in nature do get to be huge, but are also bonsai'd all the time.

I have never done bonsai but was hoping to cultivate the redwoods, maybe in pots to prepare them for being planted in the ground at the top of a stone stairway. I was hoping that they could form a sort of arbor of themselves that could be maintained at that lower 5-6 foot height. Maybe bonsai techniques could be a good way to condition the trees for the planting? Thanks again!

While you could continuously cut back the trees to keep them short, the trunks would continue to fatten, new branches would grow and the roots certainly wouldn't stop growing unless confined to a pot and also pruned quite often (considering the tree's nature).

5-6 foot dawn redwood is somewhat hugh for a bonsai and it would be difficult to get pots large enough to accomodate the size. Perhaps you may be able to integrate into your landscrape a square concrete structure of side 5 feet, preferably with the drainage holes at ground level. Any roots that grow out of the drainage holes can be lobed off to prevent excessive growth.
Alternatively, a circular structure of 5 feet diameter may be preferred.

Dawn redwood is used all the time for 'normal' (if not small) bonsai - they're not quite Giant sequoias and take very well to training and knowledgable pruning, etc.

hawk gives sound advise yet you may eventually need to trim the root structure more than wht fids its way out of the drainage. the roos will try to find open soil. you may have to dig the edges and cut about a foot off every four to five years to keep the roots contained.

Hooks, you will notice my statement: 'preferably with the draining holes at ground level'. This arrangement dispences with the digging around the edges to locate the elongated roots.
When you place an stunted tree in a pot into the ground hoping to keep it small you will fail as the plant roots will reach out of the drainage holes and eventually grow to become its natural size. If your pot is ceramic, then the roots will eventually crack the pot.