I am 5'8" 170lbs. i bought a cheap road bike with a frame material: 6061 series aluminum?

Question

I think its a 26" frame. is this a good size for my height?

Answer 1

You don't have a 26" frame you have a 26" wheel.  See the link to measure you and the bike.

http://www.coloradocyclist.com/bikefit

Answer 2

6061 is a good alloy for a frame, it's not the best, but unless you have a serious crash it should perform well and last a long time. It might ride a bit rough, but it's strong.

If it's a road bike, a good frame size for you is probably 54cm. For a mountain bike, I would think somewhere between 16.5 and 18 inches would work well.

I agree with the previous guy that 26 is probably your wheel size, but that would most likely indicate that you've got a mountain bike. Most road bikes these days have 700c wheels.