I use a PC with Microsoft PowerPoint for a lot of my design work. It uses scalable graphics rather than bit-mapped. That means you can freely scale your drawings up or down to match what you're working on.
If working from a photo, I'll insert the photo in a blank PowerPoint slide, then use the drawing tools to sketch right over the top of the original photo. I use 1/4 point line widths and make extensive use of various colors of lines, boxes and circles as layout lines.
Once completed, I select and delete the original photo from behind my artwork. "Grouping" all the individual lines and elements of my sketch into one object, enables the entire sketch to be enlarged or shrunk as needed. I'll generally make several copies on one page in different sizes and take these to the piece I'm working on to see which one fits best. Having selected a size, I print it on a laser printer transparency sheet and use a stylus to transfer this design to the cased leather.
It seems like a lot of work, but this way, even when the transparency sheet wears out from stylus abuse, I can just re-print. I can also re-use the design or even portions of the design in future artwork.