A mount must be at least one size category bigger than the rider.
Many creatures have the Strength to carry another creature of the same size category, but the game places severe restrictions on which creatures can share space on the battlefield with others (see next point).
A mount and rider share the same space on the battlefield.
For all game purposes, you and your mount function as a single unit on the battlefield. Your mount continues to use its own space and reach while carrying you. You effectively use your mount's space and your own reach (or whatever extended reach your weapon gives you) while mounted. While you're mounted, any attack that can reach a square in the space you and your mount jointly occupy can affect you or your mount (it doesn't make a difference which square). Likewise, you measure your reach for your melee attacks and the range for your ranged attacks from any square you and your mount jointly occupy. You don't get cover from your mount unless you use the Ride skill to get it, and your mount doesn't block your attacks and provide your foes with cover.
Normally, you and an ally cannot share the same space on the battlefield unless one of you is helpless or a lot bigger than the other (see page 148 in the Player's Handbook and page 29 in the Dungeon Master's Guide).
Even if a creature is one or more size categories bigger than you, it can't serve as your mount if it lacks the Strength to carry you and your gear.
http://www.wizards.com/default.asp?x=dnd/rg/20050125aRule of the Game, All About Mount......
At Least One Size Larger而已......
至于太大了以至于不能攻击...没有听谁说因为一张桌子太大人就不能站在桌子上走来走去的.......
