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Download GoSMS from the market if you don't already have it installed
(I've attached Screenshots for you to follow along)
Go to Settings, click the "Advanced" tab, scroll down and click "MMS settings"
Click on "Maximum MMS size"
Set to desired file size limit.
Then press back button
Check the box to "Preserve image resolution"
And BAM, your done. Easy Peasy.
Even though I have no 4g here in Phoenix AZ, I usually have it set to 2M. It usually takes a little longer to send on 3G but I can confirm that it does in fact send.