Of course there are plenty more elements to use width and height with, but I'll leave you to experiment with the code and try different tags out. Experimenting is really the only way to learn things - through trial and error. A percentage will increase or decrease the element to make it a certain percentage of it's parent element. Keep in mind, though, that height can not be dictated using a percentage because a pages width is comparable to screen size but a pages height can be enitirely different. Rarely are pages exactly one screen worth of info.
Units of measurement are simple enough to understand.
Generally applied to images in order to allow text to flow around them, but any element may be floated. I don't know. But I'll try my hardest to explain it. I'm assuming it is a replacement for the text wrap commands. For example, if you place an image on your page and then have about 300 words enclosed inside the paragraph tag following it then it would normally appear on two seperated lines.