"Dwarf" is relative :-) Dissectums or laceleaf Japanese maples will achieve a mature size that is dependent on where the graft is located and how the the young tree was trained. Low graft plants without benefit of staking the topgrowth will never achieve a significant height, whereas high graft specimens that have be trained/staked upright will achieve a taller tree. Low graft staked trees will be somewhat intermediate in mature height.

With the exception of species that lend themselves to coppicing or pollarding, pruning a tree to maintain a specific size is not recommended. Look for a specimen that follows the above guidelines according to your ultimate height requirements rather than selecting just any 'Garnet' maple and pruning to keep size in check. Unless you have a very skilled hand at JM pruning, I guarantee you will not be happy with the outcome.

You can reduce canopy spread to some degree but keep in mind that most weeping laceleaf forms will get wider than tall and often by a substantial margin. 'Garnet' can be expected to develop a canopy spread over time of 12-16'.