Energy-efficiency and latency requirements in alarm-driven Wireless Sensor Networks often demand the use of TDMA protocols with special features such as cascading of timeslots, in a way that the sensor-to-sink delay bound can stay below a single frame. However, this single TDMA frame should be as small as possible. This paper presents a comparative study of timeslot allocation strategies that can be used to attain this goal. The Minimum Single Frame Size Problem is formulated, and the considered slot allocation algorithms are studied based on simulations. The results point to the conclusion that informed depth-first, coupled with a longest-path-first heuristic, can improve significantly the behavior of blind depth-first. Two centralized strategies are also simulated: a longest-paths-first, which allocates the branches by decreasing order of the length of the paths, and a largest-distances-first, which allocates the branches by decreasing distances to the sink that the paths can reach. It is also shown that a largest-distances-first strategy can achieve the smallest single frame sizes, and also the lowest variation of frame sizes. A distributed version of this algorithm (DIST-LDF) is presented, which obtains the same results of its centralized version.