>The ultimate size estimates have been all over the place in my searches<

Crabapple cultivars vary tremendously in vigour, just as do regular apples. Variability of tree size is seen even when crabs are on dwarfing rootstocks. This fact makes it more difficult to manage crabs interplanted between regular trees in the orchard. In separate hedgerows or rows, the variability in vigour is not as much a problem, and can be dealt with by means of mechanical pruning.

Crabapples vary enormously in growth habit, just as do regular cultivars. The preferred growth habit is upright, in which the crab tree naturally assumes a columnar canopy shape. This canopy shape requires less space and is easier to manage. The spreading and/or weeping types should not be excluded on this basis alone, but will be more effective in planting systems where they can be mechanically pruned.

http://www.omafra.gov.on.ca/english/crops/facts/00-011.htm