In iOS6, is there a way to check how much space is being used by the Mail app?
It's quite hard and you might need to delete the mail and measure the space freed since iOS isn't showing Mail usage like it does for other apps.
In theory, you should be able to Open the Settings app and presumably:
Wait for the Storage pane to calculate the current allocation and you will see sorted from most space to least, the combined application and application data usage for each app. If Mail isn't on the list, scroll the content up and tap the "show all" entry below the initial list of apps using the most storage space on iOS.
As things stand, you will not get Mail as an entry in this list (and might spend several minutes looking for a smallish blue icon among a list of hundreds of apps).
My theory is that Apple has implemented code to prune mail storage gradually as your available space becomes low. That would mean that all Mail on iOS is a temporary cache and just like Safari pages held in cache.
If all Mail storage is simply an offline cache and not something you can depend on being on the device if you download a large game or take video and otherwise use up your storage, one viewpoint might be that the correct action is to not show the size of this cache no matter how large or small since when push comes to shove, no mail will be retained on the device if space gets low.
At best, you might need to actually save a snapshot of your usage and then delete the accounts for Mail from your device and then calculate the difference by assuming all the space freed was due to Mail storage caches.