well for the big thunderstorms they can reach to the top of the troposphere or about 12000 meters or higher. As for the size it kinda depends. the super cells might only be once city in diameter or in the tens of miles. Or the large squall lines like the one that went through the south east last night could be in the hundreds of miles. It all really depends on the conditions.
@adventuretime Could a squall line look like one big cloud? I guess you would need a satellite view, but would that be a giant cloud or many clouds scattered in a line?
And in case nobody has mentioned it, welcome to Fluther, @adventuretime!
@jaytkay yeah it would. http://www.goes.noaa.gov/browse.html the one over the northeast US is a squall line. So it really looks like one big cloud from above. But inside that cloud there are tons of different rotations, cloud heights, updrafts and other things that factor into the severity of the area. So while it looks like one big cloud it really is a line of systems all that can act differently while still moving in the same direction.