How do you change the size of the images sent from an iPhone using MMS?
One option is to add your wife's MMS e-mail address to her contact card and use that e-mail address to e-mail the picture to her phone.  When you send an image via e-mail, the iPhone will ask you what size you want to send (how much compression you want to do to the image). Choosing SMALL may allow it to go through. 
Here are a list of some SMS/MMS gateways that allow you to send a SMS/MMS via e-mail address:  List of SMS Gateways

You could also try using the free Adobe Photoshop Express app in the iTunes App Store to crop the image and save as a new image then try sending the smaller sized (both in dimensions and in bytes) image. A bit of a hassle, but the Messaging app does not have any built-in selectors for specifying the size.