There's no pre-calculated maximum bound in files exported from Spine, as those bounds can change at runtime, e.g. if you combine multiple skins, programmatically modify bone transforms etc. You'll have to loop through the animations and calculate the maximum bounds manually, possibly by sampling at discrete time intervals. This code from our Spine Web Player may be of guidance. It's using TypeScript, but should translate 1:1 to C#
spine-runtimes/Player.ts at 3.7
It calculates the maximum bounding box of a single animation.