One more thing you might consider is determining the maximum area that your light source is capable of illuminating evenly. This will determine the maximum print size without falloff at the edges, and will vary with distance from bulb to print frame. If you're handy with a spreadsheet you can do some theoretical calculations to get you in the ballpark (maybe easier to calculate if you're building with fluorescent lamps), or you might just wait and test your final setup. Here's an example of a plot from an old light box that had 6 bulbs:
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