Planning on setting up my Folger FT5 with a SmoothieBoard. From the docs I see it can handle 12.5a at 24v. So in my mind that means I should (max) be able to handle a 300watt bed heater, no? Alternatively if someone can point me to the most powerful bed heater the board supports.
12.5 x 24 is 300, yes.
Note you don't want a bed that is "just" 300W, you want one that is 12.5A at 24V ( ie with the right resistance )
However, when pushing to the absolute maximum of the board, any kind of mistake will have more dire consequence than in the "norm". I strongly recommend if you really have a need for power, you should just use a SSR. It's not very expensive, it's simple to setup, and it's much safer even at higher powers.
I have a DIY printer. My first heated bed was a 12 volt and it would not hold 110C. I looked at getting a 24 volt heater and then I needed a 24 volt power supply. So instead, I purchased a 200mm x 200mm 120 volt pcb heater. I use a SSR powered by the 12 volts of the heater mosfet. I could have used a pin to control the SSR, but this allows the leds to indicate when its heating. With the 120 volt heater, I need no additional power supply and it heats up instantly. You do have to wait a bit for the heat to penetrate you bed surface, but its faster than my hot end. I haven't been using very long, but so far its one of the best upgrades I have made.