Your casting can be as large as your kiln will allow. As a rule of thumb, the interior of the kiln should be at least twice the height of the final casting or more, depending on your setup.
For large castings, one of the most important considerations will be heating and cooling the casting uniformly.  Differences in temperature throughout the piece can result in a number of different problems, suck as thermal shock or suckers.
For examples of how to approach a large casting, see figures 9 and 10 in TipSheet 8: Basic Lost Wax Kilncasting.