Questions about weapon sizes and reach

[Expostfacto]

So I have a campaign I'm DMing and have allowed the cavalier to use a large lance two handed when mounted.

What reach would this have around the 4 squares that represent him and his mount?

What about a normal medium lance?

[Howie23]

Reach is a function of the natural reach of the user, not the size category of the weapon.

A medium rider on a large mount takes the space of the large mount. The lance would reach out 10 feet, regardless of the size of the lance.

[Bizbag]

Don't forget that while mounted, you occupy the space of your mount, so measure distances that way (that is, you can't hit an enemy adjacent to the horse to its front by saying you are sitting on the back half).