Hey,
My cages are 45"long by 16/12" high, and 18" deep, and I use the 18"x12", Pro-Product heat panel just fine. However, I don't use Vision cages so I can't comment on that type of cage. The one thing I would probably say about the material that they are made of, Vision cages (plastic), is probably that they have no insulating property, and might cause some of the heat to dissipate if the cage is not kept in a moderately warm room, i.e. 70-77 degrees. A larger heat panel will not allow for a large enough thermal gradient and might make your charges unable to escape the heat if they choose or need to.
I use melamine for my cages and they have a higher rate of insulation property, so the smaller heat panel works great - Infact too well. The heat panel I use also, has a wattage output of 65watts, so I would imagine that the larger heat panel would have a much broader thermal output rating. Try looking into the smaller unit and see what Bob says over at Pro-Products, and see what he says about it. I also, like the way the Pro-Products heat Panels mount, versus the Helix panels that Robyn prefers. He has his reasons for this though. I'll let him explain.
Hope this helps!
- Angel
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Sometimes on your way to your dream, you get lost and find a better way!