Is there a size limit or thickness limit on mailing a package media mail?

Not that I know of.  I mailed five Harry Potter books once - ohhhhhh and once a huge box of Christian books.  No problem either time.

i think it goes up to 30 pounds or 300 pieces, something outrageous like that

I just mailed a box with about 50 paperbacks to my friend via media mail.  It only cost about $6 and only took a week to travel 2,000 miles which was a surprise.  I'm moving very close to her and I'm hiding some books from DH by mailing them to her.  I'm going to mail another box this week and sneak as many as I can into my luggage when I go house hunting in 2 wks.

Post office  http://pe.usps.com/text/dmm100/what.htm says

There is a limit of 70 pounds  (costs $26.38 by media mail, btw). 
Length + girth (distance around the thickest part of the package) cannot exceed 108 inches. (Parcel Post cannot exceed 130 inches.)

The weight limit for all USPS mailing classes is 70 lbs.