disk utility disk image question

I need to backup a drive on a macbook pro.

If I use disk utility to create a "read/write" image, would I be able to boot off of that if the image is on an external USB drive?

Also how large would the writable image be in relation to the amount of space that is occupied on the MBPs drive?

Alternatively I could use super duper to backup the MBP, but that just clones the drive right? it would be nice to have an image (for data portability) (i'd like to transfer it to a time capsule as well)

What is the best way to go about doing this?

Thanks!