Spinning bike's pedals are way bigger than the cleat pedals I have. What size do I need?
Almost all adult bikes have 9/16&quot; threads, with 20 TPI.  One side is left-hand thread.
The other standard option is 1/2&quot; thread which is intended for children's bikes.
There are rare vintage standards around 10 and 12mm but your spin bike wouldn't have these.
Finally there's a chance your spin bike has some proprietary pedal thread.  Do you have access to calipers?
While pedal adapters do exist from 1/2&quot; to 9/16&quot; they are not cheap, and they separate your feet by some, also known as &quot;increasing the Q factor&quot;