Around 28-31 mm, which is rather pointless information to have, as the seat post has to have the right size down to the tenth of a mm to be useful.
27.2 mm
6 feet from post to post. And 4 feet from the bottom to the bar.
I can't even guess what you're asking about. You need a seat post of the right diameter for the frame, the right length for your needs, and preferably one suited for your type of riding. Using a road post on a MTB is not advised. If you want to get picky you can tinker around with set back and whatnot.
Depends on what's wrong with it. I'll post a link to lots of bike repair info below.
Go to www.bmxmuseum.com and post on the forums.
Yes you can but, depending on your frame, you'd need a shim to get the post to fit your frame.
See my link post to specs from the US Mint. Diameter = 19.05 mm, Mass = 2.500 g
Pretty much the same way you change any other bicycle tire. I'll post a link that tells you how.
Simply change the exponents on the bike for lighter ones, like the wheels, tires, seat post etc. this will cost more.
Remove the seat post from the frame and measure the bottom accurately with calipers.
Usually simply by opening the seat post clamp and sliding the seat post down. Sometimes by either cutting the post down to make it shorter or by buying a shorter post.
You can use pretty much any disc brake wheels, any handlebar with an oversize mid section, any seat post with the correct diameter, any saddle, any tire with a 559 mm BSD, etc.