you can do this with doubling or tripling, for sure...but always work with the finished size of the block and add seam allowances after you figure out the new finished dimensions. I've done it several times. When you enlarge HST's or QST's you're dealing with some odd dimensions, so I'd suggest making them larger than you need and cut them down to the size you need rather than trying to deal with exact dimensions to begin with. I'm less sure about just adding inches to do the increasing...it should work, but just not positive...