Short answer--you can't.
Regarding the 25-message limit: You can change this by going toi Settings -> Mail and change the number of messages to show; as you delete messages, more will be downloaded.
I've gone through this painful process more than once, following updates and restores when my iPhone wants to download all the GMail I've received in the last 30 days (about 5-6 per day) after the restoration; and I'd like to see deleting multiple messages in some (near? please??) future update.