I've been using this edited MMS.apk. It allows 1000k messages. Just go to system/app and push it with adb or root explorer. Rename the original mms.apk.bak or something. Copy in folder, install and reboot.
Yes, I'm on Droid X MIUI 1.8.19 and i've been trying to get a way to work this out. I've tried myself and no go. The settings menu allows you to change the limit of deleted messages not the size limit of messages sent. Someone posted an MMS.apk app but it's been removed from rapidshare. Is there a way to get this again? On a newer version. All other messages apps allow you to change the size limit, but MIUI won't. It's really driving me crazy switching back and forth between 2 messaging apps. I just want to use "ONE".