Can I use the character animations in the starter content for a custom character? If so, what do I have to keep in mind when making the biped skeleton of my character (number of bones, bone size)?
You sure can! You’ll just want to make sure your custom character has the same bone hierarchy and bone names. It doesn’t need to be the exact same hierarchy though. For example if you only needed 3 fingers instead of 5, that would be fine as long as those 3 fingers shared names with fingers in the animation skeleton. You can also add bones to the end of existing joints, like a ponytail. That would also be fine. Just the core hierarchy needs to stay the same and you’ll be good.
You can actually export the skeleton that those animations use from the engine to an fbx. Just right click on the mesh those animations are mapped to and select export to fbx. this will give you the best starting point.