I had missed the fact that the Hero does in fact automatically scale down photos for sending them in MMS picture messages.
There must therefore be a software routine somewhere in the phone to do this resizing. It seems a shame that the routine is not accessible when emailing a photo.
However, googling elsewhere seems to confirm that there is no built-in way of resizing for email. Luckily, there is an app called PicSay which does the job very well. The free version saves to a size suitable for mobile phone screens. The Pro version ($1.99) has other size options.
