A poster is 20 by 30 in inches. which one is the length and which one is the width.?

The length is the longest dimension, so that would be 30", therefore the width would be 20". See the reference for more information. Now if the poster hangs with the 30" side horizonally, you can call that the width, but then you'd have to call the other dimension the "height", not the "length." Also : In construction the width always comes before the height.