On a Brava/o, the strut towers are located quite close to the bulkhead, making the engine bay stiffer than many other cars anyway, so the strengthning benefits of a strutbrace are'nt as noticeable as on say, a Mk3 Escort.
That's not to say one would'nt help - but in reality, few road cars are driven hard enough, (or on bad roads) to justify one.
As has been said, the main point is to minimize strut/enginebay flexing to keep the front suspension geometry as 'true' as possible during extreme corning/braking.
The car's engine size is'nt so relevent - only in that a heavier engine/greater torque could put more strain on the engine bay structures, (if driven hard) - together with the usual assumption that a bigger-engined/sportier version car is going to be driven harder than a smaller version.