Ah, yeah, I've come across this problem before. UTAU won't change the length of super duper long notes by dragging to change the length; what you have to do is right-click, hit 'length', and enter how long you want your note to be. A quarter note has a length of 480, so multiply that by however many quarter notes long you want it to be and put it in the box. (Another option is entering a low number such as 480, then the note will be a lot shorter and then UTAU will let you change its length as you would normally.)
Hope that helped! ^^