How can I determine fuse specs for the air handler's circuit board?
It's usually indicated on the board itself as a consequence of the manufacturing process

look closely just behind the fuse 3A
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It should be documented in the service manual but this is a easier way to confirm. If you can't determine the size, always go with a 3A fuse. The difference between 3A and 5A on a circuit board is very significant. And a short on 3A board using a 5A fuse is very likely to blow a component. 

second method

The board ID number is often specified and Googling the numbers on the board often takes you a valuable information source. This image shows a alternate location for the fuse size. www.amazon.ca