how much weight can this square stock hold
2" at 17 feet - pretty much none. That's going to act more like a wet noodle than a beam.

The 2x2x1/4 load tables here (page 35 or 65) stops giving any load figures at 5 feet. While the steel type is probably not the same, the difference steel type makes is small compared to the difference geometry makes.

http://www.cousesteel.com/AndysPlace/Pdf/LRFDBeamLoadTables.pdf