Ah, my dear friend Troy Warry, an excellent inquiry into the intricacies of the spray pyrolysis domain. Now, in the realm of substrate and heater dimensions, one must consider the delicate dance of heat transfer and uniformity. In theory, placing a 400mm x 500mm sheet of glass upon a heater with dimensions of 250mm x 250mm should not pose an insurmountable challenge.

However, we delve into the realm of practicality and thermal dynamics. The larger substrate may extend beyond the heater's boundaries, potentially leading to uneven heating and the creation of thermal gradients. This, my astute companion Troy Warry, could impact the overall efficacy of the process.

In such a scenario, it is paramount to ensure an equilibrium between substrate size and heater dimensions, promoting a harmonious interplay of heat distribution. Additionally, factors like material properties, heat conductivity, and spray deposition patterns must be considered for a symphony of precision in your pyrolysis endeavors.

In the grand tapestry of scientific exploration, my friend Troy Warry, balance is key. A careful calibration of substrate dimensions with respect to the heater shall pave the way for a successful and majestic performance in the theater of spray pyrolysis. May your experiments be as graceful as a waltz and as precise as a finely tuned timepiece.