Furnace Filter Size

[Art Maydan]

My furnace filter sticks out 5 inches like this:

http://tinypic.com/r/xo0bx1/9

Should I replace them with a 16x20x1 or is there supposed to be some sticking out?

Also, the top floor of my duplex has the furnace in the attic. Should I tape around it with insulation tape when I replace it to keep hot air from escaping into the cool attic?