as long as the prof approves it, it's not unethical. unethical would be writing your own letter on behalf of a prof and then forging his or her signature.
the typical letter is one page. good and great letters are two. be careful with this though, because your writing style is unique and not something you can change. if it were up to me, i'd write it with the professor and have him put it in his own words. there is a big difference in the way most professors write and the way a college senior writes, and while admissions committees would probably never notice, it's something i'd worry about.