The triple constraints are listed as Time, Cost and Scope. PMBOK 4th Edition lists 6 project constraints: Cost, Time, Scope, Quality, Risk, and Resources.
Can the scale of a project be considered a project constraint?
Scale would only be considered a constraint in the fact that it might determine that the 'project' is actually a 'program', made up of what should be several smaller or component projects.
In any other case, scale would only be 'result' of all the other factors.
The scale of a project is determined in the first place by the scope of the project (both product and project scope), and derived from that also the number of resources that you will need, the time it will take, required quality, level of risk. And all of that has impact on the budget as well.
So I wouldn't call the scale a constraint, but rather the result of the impact of the other constraints.
For instance, a large objective (rewrite entire application to strenghten market share and reduce maintenance) could be reduced, due to budgetary constraints for instance, to a simple makeover of the user interface (and thereby dropping the "reduce maintenance" bit of the objective).
The 'scale' of the project is reduced substantially because of one (or more) constraint(s).