Many things that we have on the SPRINT side of things are disabled or limited BECAUSE SPRINT ROM's have certain things which other ROM's do NOT and therefore, sometimes limit our abilities to do things... At thesame time, disabling/changing these settings give us almost unlimited usability 
... jejeje
Anyway, when sending MMS, SPRINT DOES NOT HAVE A LIMIT on the network (like AT&T has, VERIZON too) but they put limits on THE PHONE itself which causes the "LIMIT" to appear to be a NETWORK issue... BUT ITS NOT!!! 
Took me a WHILE to find this registry setting but I FOUND IT!!! jejeje [I AM CURRENTLY RUNNING THE TANGO V1 8773, by ANSAR]
Go to the REGISTRY EDITOR and navigate to this entry:
HKCU\Software\Microsoft\ShortMsg\MMS\Profiles\Spri nt\MS
The MS entry should have a "Value Data" of "2000", change this to "51200" press "OK"
You should now see the MS entry have "0x0000C800 (51200)"
Ohhh Yeah baby, thats it!!! 
You are done my friends... Go wild sending multiple videos and pictures because the size should not be an issue anymore!!! ... jejeje
I am almost certain this will work with other ROM's as long as they are for the GOLD_C (HTC Arrive) but I have not tried it on another yet...
***IF YOU ARE LOOKING TO REMOVE THE 160SMS CHARACTER LIMIT, SEE MY OTHER THREAD:***
I CANT STRESS THIS ENOUGH: THANK YOU ANSAR FOR THE CUSTOM ROM AND ALL DEVELOPERS THAT HELPED UNLOCK OUR PHONES
AND OF COURSE, DONT FORGET TO HIT THE "THANKS" BUTTON IF THIS HAS HELPED YOU!!! JEJEJE